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
empty_value translation
sonata_type buttons changes
Merge pull request #1587 from tiagojsag/sonata_type_buttons

Added custom catalogue for sonata_type buttons
improvements to install docs
Merge pull request #1588 from tiagojsag/install_docs

improvements to install docs
Fix #1446
Merge pull request #1581 from tiagojsag/empty_value_translation

empty_value translation
Merge pull request #1467 from tashanta/master

Fix form creation url when object id is 0 (Issue #1466)
Merge pull request #1465 from joostfarla/getAdminsByGroup

Simplify retrieval of admin groups
Merge pull request #1453 from javiern/patch-1

Update ModelToIdTransformer.php