Add stash commands

I'll need this for GitX soon™, but I'm not yet using it, so I'm unsure if that API is final or not. This is based on #163 for the "create stash" part.

Wonderings :
- I'm unsure about the `enumerateStashes:` call block format. Right now it's a simple wrapper around `git_stash_foreach`. Maybe something more convenient can be whipped up, but since I'm not yet using it I have no idea ;-).
- I know I'm doing strange things with the `const GTSignature.git_signature` pointer. But I'm unsure how to fix that (short of changing `git_stash_save` too). Opinions ?