Merge pull request #36 from sylvaindeloux/master

SONATA-455 - Added Select2 assets
Add missing translations for EqualType

File 'Sonata/CoreBundle/Form/Type/EqualType.php' lines contains:
42: self::TYPE_IS_EQUAL => $this->translator->trans('label_type_equals', array(), 'SonataCoreBundle'),
43: self::TYPE_IS_NOT_EQUAL => $this->translator->trans('label_type_not_equals', array(), 'SonataCoreBundle'),

'label_type_equals' and 'label_type_not_equals' are used and are missing in translation files.
I copied them from SonataAdminBundle.
Merge pull request #37 from mieszko4/patch-1

Add missing translations for EqualType
Updated doc & unit tests
Merge pull request #40 from Bladrak/updateDoc

Updated doc & unit tests
Added BaseSerializerHandler