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)

So something like this diff then?

```diff
@@ -455,9 +456,13 @@ def get_credentials(fn, url, username, allowed):
 
     elif cred_type == C.GIT_CREDTYPE_SSH_KEY:
        name, pubkey, privkey, passphrase = creds.credential_tuple
-        err = C.git_cred_ssh_key_new(ccred, to_bytes(name), to_bytes(pubkey),
-                                     to_bytes(privkey), to_bytes(passphrase))
-
+        if pubkey is None and privkey is None:
+            err = C.git_cred_ssh_key_from_agent(ccred, to_bytes(name))
+        else:
+            err = C.git_cred_ssh_key_new(ccred, to_bytes(name),
+                                         to_bytes(pubkey), to_bytes(privkey),
+                                         to_bytes(passphrase))
     else:
         raise TypeError("unsupported credential type")
 
```