Added git_stash_apply() and git_stash_pop() APIs

This is a tentative implementation of git_stash_apply() based on the actual Git implementation found at https://github.com/git/git/blob/master/git-stash.sh#L440.

Once you have git_stash_apply(), it's trivial to implement git_stash_pop() so I also added it.
Hey, I just copied exactly what you did for `git_stash_save()` :) Seriously though, default means no options. You can't really describe that. I guess `DEFAULT` is just a syntactic convenience for `0`, I was just following your pattern. I don't care if people pass 0 or `DEFAULT`.

Just give me the description for `DEFAULT` for `git_stash_save()` and I'll model mine on yours.