Made email verification service configurable at the form field level

- Introduced a parameter 'email-verification' in CM_FormField_Email
- Overrides the default (config based) email verification service
- Added tests
Merge branch 'issue-1402' of github.com:fauvel/CM into issue-1402

Conflicts:
	library/CM/FormField/Email.php
	tests/library/CM/FormField/EmailTest.php