Support for class-level validation

This allows for

    $errorElement->addConstraint(new UniqueEntity(['fields' => 'email']));

and similar code without an enclosing ->with().
Update ModelToIdTransformer.php

This is to fix problems with entities with 0 as id, empty will return true if u give 0 as parameter
adding unit test for ModelToIdTransformer in the cases where it gets 0 as id
removed test class from the file
Implemented getAdminsByGroup() / hasGroup() functions
Test if identifier is null or not in twig templates, to determine if we have to generate an edit/create form

Fix Issue #1466 create/update routing problem in form creation, when id is 0
Fix correct id in aclAction
Fixed minor typo in translation.rst
Made 'Admin'/'Admins' the consistent style for referring to a SonataAdmin Admin class
Updated console command docs and tweaked GenerateAdminCommand:
* expanded sonata:admin:generate docs
* switched order of list and explain in docs, since you need to list first before you know what to explain
* refactored GenerateAdminCommand::getAdminServiceId() so it only strips the end off Bundle names that end in "Bundle"
* edit for readability
Updated console docs:
* added short note about sonata:admin:generate-object-acl
* updated inline documentation in GenerateObjectAclCommand.php
Updated advance.rst:

* fixed table formatting (old version was hidden)
* fixed 'security_feed' typo, the example is actually about 'project'
* moved the RouteBuilder customisation info to its own sub-section
* moved the generic customisation text above RouteBuilder
* tidied up the generic customisation example
* tidied up comments in sort-order section so they should display better (without line wraps) on sonata website
Cleaned up and expanded batch_actions.rst:
* added note about two options when adding a new batch_action
* added note about what the merge example is trying to do and when it's available
* edited code blocks to prevent line wraps on sonata website
* updated list__batch.html.twig example to reflect latest contents
* added link to Symfony bundle extension docs
Updates to security.rst:
Fixed indentation issues which were creating rogue DLs and list levels
Minor spelling/grammar edits (unlike French, English has no spaces before colons)
Add missing test on camelize
Fix boolean definition for options in configuration array
Merge pull request #1532 from caponica/docs-20130808

Updated docs and some related console commands
Add admin_pool test
Update Preview Mode Documentation

Added more code examples and some screenshots, also
cleared up text a bit.
Fixed typo
Added links to new action_xyz docs in index.rst