Allow 'bit' field in MySQL structure

This change will allow you to create a `bit` field using the `Gdn_DatabaseStructure` class.

This change also does a `strtolower` on the column type before making the check so that you can specify field types regardless of case - perhaps this is by design, though.  If that's the case, I can remove that change from the pull request.

This is a repost of #2233  against staging.