Pdpyras

* Fixed regression bug / departure from documentation (17): the ``payload`` parameter does not merge with but rather completely replaces the default payload

* Changed behavior of HTTP retry that caused issues with some internal tools: raising ``PDClientError`` in the event of non-transient HTTP error, in the ``request`` method, versus returning the request object and logging it. The previous behavior was:
* Not the intended design
* At odds with the documentated behavior

* 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.