Added getShortname for reference

Hey,

I'm not sure if there is a better way to do this or if this covers all use cases, I needed a way to grab a short name from a reference the ref/heads/ and ref/tags/ is meaningless to the end user. So I made a quick function.

As I said before I'm not sure if this is the best way to do it, I'll leave that up to you guys to decide. I figured I'd at least start a discussion on it.