Snapshot support

[carnage]

Pulling out events numbering in the 100's -> 1000's to reconstruct an aggregate root is fairly inefficient.

Provide support from the repository class to snapshot data using a defined set of interfaces.

Implement an in memory implementation of this set of interfaces for testing purposes as part of this lib

Implement snapshoting using Doctrine 2 ORM in the cqorms module. => This could lead to VERY rapid performance if the SLC is used to cache snapshots. (They should always be read safe as any snapshot will be brought up to date with any new events from the event store before it's returned)