Es-stats

**Changes**

* Add ``nodeid`` as a keyword arg to the ``stats`` method of each child
class. This allows the stats of any arbitrary nodeid to be pulled.
* Put the classes in ``__init__.py`` to allow them to be more easily reached.

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**Changes**

* Renamed ``cache_frequency`` to ``cache_timeout`` as `frequency` didn't
make as much sense.
* Renamed ``_version.py`` to ``version.py`` for project consistency's sake.

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**Bug Fixes**

* Refactor how the instance attributes ``nodeid`` and ``nodename`` are
initialized, and treated when/if a node name is passed to a child class.

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**New**

* Update to only get the `_local` node information at initialization time and
set the instance attributes ``nodeid`` and ``nodename`` from the `_local`
node. The previous behavior was to just take the first in the list.
* Allow any ``elasticsearch-py`` version >= 5.5.2

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**Bug Fixes**

* Fix incorrect version in ``_version.py`` for PyPI release

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**New**

* Redundant code removed by making classes children of a parent class,
``Stats``.
* Caching of stats calls. Configurable with ``cache_frequency`` kwarg in
class initialization. This should make repeated calls to crowded APIs
like NodeStats much faster, without hammering the cluster. Default value
is 60 seconds.
* Improved linting helped clean things up. Removed many unnecessary import
statements.
* Added ``quicktest.py`` which should test against a local, unencrypted node.

**Breaking**

* Cluster health used to be mapped in this module from green/yellow/red to
0/1/2, respectively. This is no longer done here. It will respond with
``green``, ``yellow``, and ``red`` as the API does. For those of you who
are Zabbix users, the ``es_stats_zabbix`` module will handle the change, as
it should be mapped on that end.