Add CreateOrphanBranchCommand

This isn't my work - it's almost a direct copy of https://gist.github.com/taichi/3097455/

I used this code locally and it works great for me, and some googling suggests this is a common question with jgit and the solution is not obvious - so I think this was valuable work that should be part of the jgit API.

I added the CreateOrphanBranchCommand and its test class. Also updated Git.java to add a convenience method for creating a CreateOrphanBranchCommand.

The version of this command in the gist introduced javax and guava dependencies. I removed the javax, but kept guava.

I wasn't able to get the linkFail or toBeDeleted tests working.. it wasn't clear what was needed to get these tests working.