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)

One of these lies is redundant. If we're going to rely on the superclass' constructor, then there's no need for us to store the username explicitly.