authenticate using ssh agent

Introduce the ability for pygit2 to use the SSH agent for authentication.

Example use:
import pygit2
x = pygit2.credentials.SSHKeyFromAgent('git')
r = pygit2.clone_repository('ssh://git@github.com/some-org/some-repo', '/tmp/something', credentials=x)

We thought about this and weren't sure which way to go. The only thing useful we could detect is `tuple[1:]` are all None. We went with this way as this is what the Ruby libgit2 bindings do.

Happy to change this to trigger off pubkey & privkey being None instead if you'd prefer.