Laterpay-client

* Fixed encoding issues when passing byte string parameters on Python 3
([84](https://github.com/laterpay/laterpay-client-python/issues/84))

* Added support for Python requests >= 2.11
* Added current version number to HTTP request headers

* Enabled universal wheel builds

* `laterpay.utils.signed_query()` now passes a `dict` params instance to
`laterpay.signing.sign()` which makes it compatible with `sign()` ignoring
some params being ignored by `sign()` by looking them up with `in` operator.

* Added Python 3.5 support

* `LaterPayClient.get_add_url()` and `LaterPayClient.get_buy_url()` accept
`**kwargs`.

* The parameter `skip_add_to_invoice` in `LaterPayClient.get_add_url()` and
`LaterPayClient.get_buy_url()` is deprecated and will be removed in a future
release.

* `laterpay.signing.sign_and_encode()` is deprecated and will be removed in a
future release. Consider using `laterpay.utils.signed_query()` instead.
* `signing.sign_get_url()` is deprecated and will be removed in a future
release.
* `LaterPayClient.get_access()` is deprecated and will be removed in a future
release. Consider using `LaterPayClient.get_access_data()` instead.
* `LaterPayClient.get_identify_url()` is deprecated and will be removed in a future
release.
* New utility functions: `laterpay.utils.signed_query()` and
`laterpay.utils.signed_url()`
* New `LaterPayClient` methods:

* `get_request_headers()`
* `get_access_url()`
* `get_access_params()`
* `get_access_data()`

* Improved and newly added tests.
* Improved `laterpay.signing` docs.
* Replaced most of `compat` with proper `six` usage.