Pull request for bb-1153 (please review)

I added jquery-validator for validating the emailaddress on-the-fly when logging in. It has separate message files per language, I conditionally load the dutch message file but you might want to handle this differently (I'm not sure how we're currently handling JS translations).

* adding warning when user doesn't enter an emailadres
* using type=email fields instead of type=text
* adding mention of logging in with emailadres in error message