Set return tag on field if documentation lacks it
Document ActiveRecord Validations

First pass at adding validation information onto each fields definition.

I'm expecting to discover additional corner-cases that this doesn't handle
as it sees additional use.
Turn ActiveRecord references into links
Validations handler

This adds links to the 'validates' handler.

It links to the docs for the standard links and custom types can be added by calling
YARD::Handlers::Ruby::ActiveRecord::Validate.add_validation_type( type, link )
Use recommended register usage

Loren Segal (yard creator) pointed out
(https://github.com/lsegal/yard/issues/758#issuecomment-38754867)
that register was used incorrectly, causing comments to over-write
the generated method.
Merge pull request #19 from nathanstitt/develop

Add validations handler and fix various bugs