Add checkboxesField{,List} fields (based on multiSelectField{,List})

Issue #602 https://github.com/yesodweb/yesod/issues/602

I think we should have the choice between select tags and input type=checkbox tags for a multiple selection field in a form. Here I've made an adaptation based on multiSelectField{,List), it only change the view implementation (see code below).

I was wondering why isn't there already this implementation? Is there any suggestion about this? We may factorize some helpers function to avoid duplicating code (like selOpts).

PS: the field was renamed, the multiplicities of checkbox(es) is already implied, no need for 'multiple'.