Added keyword argument, default_opts, to rsync_project.

Previously there was no way to remove any of the predefined rsync options. This
change allows them to be modified via a keyword argument.

This does seem to make the extra_opts argument slightly redundant. I could remove it, but there would be backwards incompatibilities.

I also don't understand why the full call signature (all args) doesn't render in the Sphinx docs for rsync_project. The docs and usage would be clearer if the call signature didn't revert to:

fabric.contrib.project.rsync_project(*args, **kwargs)

Instead rendering as:

fabric.contrib.project.rsync_project(remote_dir, local_dir=None, exclude=(), delete=False,
        default_opts='-pthrvz', extra_opts='', ssh_opts='', capture=False,
        upload=True)