test utils: implement a repo context manager

The current hierarchy of test cases that create and blow away test
repositories in the ``setUp`` and ``tearDown`` methods is inadequate
for testing scenarios where multiple repositories must be edited,
e.g.  testing push.

Extract the temporary repository behaviour into a context manager
class ``TemporaryRepository`` which can be used via the ``with``
keyword.  The context manager's ``__enter__` method returns creates
the specified repository in a temporary directory and returns its
path.  The temporary directory is removed in ``__exit__``.

Update the existing test case base classes to use the context
manager and invoke ``__enter__`` and ``__exit__`` in the ``setUp``
and ``tearDown`` methods respectively.

Finally, make some small tweaks to a handful of tests to get them
working under this new system.
implement push support

Implement push support via Remote.push which is called with a single
refspec and raises GitError (with an appropriate message where
possible) if the push fails.

Note that local push to non-bare repository is currently not
supported by libgit2.
Doc fixes: change head.oid to head.target in examples
Merge remote-tracking branch 'fraser/feature/push'