Optionally return tag information from feed API

As discussed in the Google group (https://groups.google.com/d/topic/gitblit/QPsBzhAMvB8/discussion) this adds the capability to return tag information from the feed API instead of commit details.
This adds an optional 'ot' request parameter, which if set to TAG will cause details of all the tags in the repository to be returned instead.

This seems to be working for me, but I am pretty new to git, so let me know if there are any issues and I can try and fix them.

Many thanks,
Gerard