[documentation] Added description of Role Handler

There was a lot of doc about ACL, but (nearly) nothing about the role handler. So here is some precisions about it.
Commented about access_strategy
Updated slovak translations
Added ability to set translation domain on field description
Moved [get|set]TranslationDomain to interface
Allow method chaining for add methods
Merge pull request #1371 from joostfarla/method-chaining

Allow method chaining for add methods
Merge pull request #1363 from pulzarraider/patch-2

Updated slovak translation
Refactored docs

- Added more structure
- Added section on setting translation domain for fields
Merge pull request #1365 from dantleech/ext_trans_domain

[WIP] Added ability to set translation domain on field description
Updated doc
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
Merge pull request #1359 from romaricdrigon/patch-1

[documentation] Added description of Role Handler
RoleSecurityHandler now use groups; cf #1146
Added tests for RoleSecurityHandler