Pdpyras

* Introduction of a custom ``User-Agent`` header to distinguish the API client as such, for the purposes of usage analytics

Important bug fixes to the custom HTTP retry logic:

* Fixed ``KeyError`` in ``APISession.request``
* Fixed incorrect behavior (retrying more than the specified amount of times) due to faulty comparison logic

A light Events API client methods refactor:

* All keyword arguments specific to sending trigger events have been refactored out of the generic ``EventsAPISession.send_event`` method
* Now, instead, ``send_event`` and uses a catch-all keyword argument to set event properties.
* The keyword arguments specific to triggering incidents are in the method EventsAPISession.trigger method.

* Added new Events API session class that still has most of the same functional features as the REST API session class.

* Fixed bug: unpacking wrapped entities does not work with ``/log_entries``

* Whitelisting of endpoints supported by the ``r*`` / ``*_all`` methods has been rescinded, and documentation has been updated with how to identify endpoints that these methods can be used with.