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)

I'd rather see this choose based on the values returned from `.credential_tuple` rather than based on inheritance. It's not too big a deal, but the idea behind these is that each instance tells us about what it's doing, rather than rely on the type.