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)

Yeah, that's how I'd go. This is designed so you can use your own classes which get the data from wherever you need (disc, the user, a database, whatever), and there's no need for them to hang off of these classes, all they need to do is have this attribute with the data we need.