Event item refactor & Acceptance Tests

[mumby0168]

Introduced some acceptance tests

1. Tests the persist methods
2. Tests the read methods
3. Tests the change feed processing to build projections
4. Used a single test since the startup time of the library to create containers and DBs etc takes around 20 seconds so running multiple tests just takes longer.

Refactored the DefaultEventStore<T> to use the specific interfaces provided by the repo.

Updated docs to reflect changes to EventItem. This imp of EventItem makes serialization behave better and does not force the consumer to follow any specific ctor implementation.

[IEvangelist]

Looks good to me, when you're ready.