Merge pull request #40 from baldurrensch/serializer_0.12

Update to JMS Serializer 0.12
Update readme to tell to require 0.4
Add contributors to the author list
Add @baldurrensch to the author list :-)
skipIfNull parameter in @Relation
Renamed skip_if_null to exlude_if

- Now checks for the value and skips
- Added unit tests