Include libgit2 headers in our module

_Supersedes #399._

This builds fine, but when I try to import ObjectiveGit in a project, I get:

```
Could not build module 'ObjectiveGit'
```

And that's the extent of the error message. :crying_cat_face: 

I've also tried creating a separate libgit2 module but wasn't able to get Xcode to actually see it.