Add validations handler and fix various bugs

This adds a handler for validations, and fixes the following bugs:

 * The see also links where not links because the get_tag method didn't set the name on the tag.
 * If a comment was already present on an association, the generated method's parameters were ignored, leaving the documentation without a return type and the "see also" links.
 * If a comment was left on an model's field by using the @attribute tag, the field_handler would not document the field, which would leave the return type blank.

I'm sorry for the rather large pull request.  I'm using yard & yard-active record to document my Stockor project  https://github.com/argosity/skr-core & http://stockor.org/core/doc/.  I set out to just add the validations and then also encountered and fixed the above issues.

Normally I'd prefer to have split the work into separate branches and submit each component separately.  This kind of just kept growing and I never split the work apart.  If you'd prefer not to have any of the above in the code base, let me know and I can rework this.