Add snapshots

[lukaszb]

Currently Repository.get_aggregate method is not very efficient - it fetches all events that were used to create an aggregate. In order not to do that we want to introduce snapshots.

Change how aggregate is retrieved

Logic behind the get_aggregate should be changed as follow:

• fetch snapshot with greatest version number for particular aggregate (or all events if there are no snapshot yet)
• fetch all events with version greater than that from the snapshot

Store snapshots

• Add Snapshot model
• Add Storage.create_snapshot method
• Create snapshot "every now and then" (i.e. every 20 events - this variable should be controlled somehow)