Merge pull request #1376 from dantleech/ext_trans_domain

Updated doc
fix deprecated block name
Replaced deprecated DataEvent and FilterDataEvent with generic FormEvent
Replaced deprecated events PRE_BIND and BIND with PRE_SUBMIT and SUBMIT
Reverted last commit to preserve compatibility with Symfony 2.2
Fix error with form
Added basic support for setting group translation_domain

- Ability to set translation_domain on a form group
- Added unit test for FormMapper
- Extended AdminInterface to have [get|set]FormGroups