Implemented aggregate versioning as a prerequisite for coarse-grained locking
Updated sequel gem to 4.6
Stripping :_version in BaseEntity.full_update
EntitySerializer.to_hash* methods support skipping classes from serialization
Merge remote-tracking branch 'origin/master'
Fixed: version not being updated when inserting new child
Implemented pessimistic concurrency. Closes #18
LazyEntityReference does not resolve its enclosed entity in order to get its version
Fixed: id:nil was being inserted in _version
Fixed: commited objects in STATE_NEW weren't being locked