ObjectiveGit 0.1 and libgit2 0.17.0

Specs for libgit2 0.17.0 and ObjectiveGit 0.1. 

ObjectiveGit 0.1 needs an untagged version of libgit2, so I added that version, too.

There is an existing objetcive-git 0.0.1 already in CocoaPods, but 
I couldn't manage to build the 0.1 version with that name.

This is my first try of a spec, so please don't be too hard with me. :)