Client Update - Requires a new release tag

Adds methods to the client for `addEvent` and `addEvents`.

This removes the need for a user to pass their event data nested under the `data` property in the method calls.

Renaming `data` to `keen_io_event` / `keen_io_events` in the service description for `addEvent` and `addEvents` to better namespace/avoid collisions down the road.

Added improved / additional Unit Testing. Documentation update and added basic change log doc. Minor CS fixes.