Update "Form type extension" cookbook for Symfony 2.2

Some deprecated calls where made and have been replaced by their newer counterparts.
Merge remote-tracking branch 'origin/fix/create_form_extension' into 2.3

Conflicts:
	cookbook/form/create_form_type_extension.rst
Merge pull request #376 from Greenflex/2.3

Update "form type extension" cookbook for Symfony 2.3