Pushjack

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- gcm: Add support for ``priority`` field to GCM messages via ``low_priority`` keyword argument. Default behavior is for all messages to be ``"high"`` priority. This is the opposite of GCM messages but mirrors the behavior in the APNS module where the default priority is ``"high"``.

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- gcm: Add support for ``notification`` field to GCM messages.
- gcm: Replace ``registration_ids`` field with ``to`` field when sending to a single recipient since ``registration_ids`` field has been deprecated for single recipients.

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- gcm: Fix incorrect authorization header in GCM client. Thanks `Brad Montgomery`_!

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- apns: Add ``APNSSandboxClient`` for sending notifications to APNS sandbox server.
- apns: Add ``message`` attribute to ``APNSResponse``.
- pushjack: Add internal logging.
- apns: Fix APNS error checking to properly handle reading when no data returned.
- apns: Make APNS sending stop during iteration if a fatal error is received from APNS server (e.g. invalid topic, invalid payload size, etc).
- apns/gcm: Make APNS and GCM clients maintain an active connection to server.
- apns: Make APNS always return ``APNSResponse`` object instead of only raising ``APNSSendError`` when errors encountered. (**breaking change**)
- apns/gcm: Remove APNS/GCM module send functions and only support client interfaces. (**breaking change**)
- apns: Remove ``config`` argument from ``APNSClient`` and use individual method parameters as mapped below instead: (**breaking change**)

- ``APNS_ERROR_TIMEOUT`` => ``default_error_timeout``
- ``APNS_DEFAULT_EXPIRATION_OFFSET`` => ``default_expiration_offset``
- ``APNS_DEFAULT_BATCH_SIZE`` => ``default_batch_size``

- gcm: Remove ``config`` argument from ``GCMClient`` and use individual method parameters as mapped below instead: (**breaking change**)

- ``GCM_API_KEY`` => ``api_key``

- pushjack: Remove ``pushjack.clients`` module. (**breaking change**)
- pushjack: Remove ``pushjack.config`` module. (**breaking change**)
- gcm: Rename ``GCMResponse.payloads`` to ``GCMResponse.messages``. (**breaking change**)

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- apns: Add new APNS configuration value ``APNS_DEFAULT_BATCH_SIZE`` and set to ``100``.
- apns: Add ``batch_size`` parameter to APNS ``send`` that can be used to override ``APNS_DEFAULT_BATCH_SIZE``.
- apns: Make APNS ``send`` batch multiple notifications into a single payload. Previously, individual socket writes were performed for each token. Now, socket writes are batched based on either the ``APNS_DEFAULT_BATCH_SIZE`` configuration value or the ``batch_size`` function argument value.
- apns: Make APNS ``send`` resume sending from after the failed token when an error response is received.
- apns: Make APNS ``send`` raise an ``APNSSendError`` when one or more error responses received. ``APNSSendError`` contains an aggregation of errors, all tokens attempted, failed tokens, and successful tokens. (**breaking change**)
- apns: Replace ``priority`` argument to APNS ``send`` with ``low_priority=False``. (**breaking change**)

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- apns: Improve error handling in APNS so that errors aren't missed.
- apns: Improve handling of APNS socket connection during bulk sending so that connection is re-established when lost.
- apns: Make APNS socket read/writes non-blocking.
- apns: Make APNS socket frame packing easier to grok.
- apns/gmc: Remove APNS and GCM ``send_bulk`` function. Modify ``send`` to support bulk notifications. (**breaking change**)
- apns: Remove ``APNS_MAX_NOTIFICATION_SIZE`` as config option.
- gcm: Remove ``GCM_MAX_RECIPIENTS`` as config option.
- gcm: Remove ``request`` argument from GCM send function. (**breaking change**)
- apns: Remove ``sock`` argument from APNS send function. (**breaking change**)
- gcm: Return namedtuple for GCM canonical ids.
- apns: Return namedtuple class for APNS expired tokens.