Updated IMemento implementation to hide some of the details from end users
Updated AggregateBase to generate an 'EventSourceId' which is just Id.ToString
Removed AggregateString and AggregateGuid in favor of AggregateBase<String> and AggregateBase<Guid> again

Probably the beginning of a complete rewrite.  I am planning on removing EventSourceMapper and changing IUnitOfWork to be more pronounced
See comments on issue #33