Adds methods to the client for `addEvent` and `addEvents`.

This removes the need for a user to pass their event data nested inside
an array with a ‘data’ key.

Renaming `data` to `keen_io_event` / `keen_io_events` in the service description to better
namespace/avoid collisions down the road. In hindsight, `data` was too common of a property
name.

Added additional Unit Testing.

Minor CS fixes.

Documentation update and added basic change log doc.
Updating documentation
fixing doc formatting
Merge pull request #34 from rebeccastandig/master

Updating documentation, fixed doc formatting.
merge new documentation changes
Merge pull request #33 from kmfk/bc-break

Client Update - Requires a new release tag
version in composer to 2.0
Fixed reference to change log