Some more Status API.

Inspired by the few direct libgit2 accesses which don't have Objective-Git counterparts left in the GitX code.

ccfc833 is mostly under "development". This commit adds the parts referred to by #241, but I'm not sure what's the "cool" way of handling errors while returning meaningful results, as passing pointers around doesn't feel very Obj-Cy. Comments welcome.

I would like to do some quick profiling on 6f23287 too, because I'm not sold yet ;-).