Fix MacOSX backend installation issues.

As reported by @mdehoon:

> I noticed that this release candidate will always skip the Mac OS X backend,
> as the .check() method of BackendMacOSX in setupext.py returns None. Adding
>     return "darwin"
> or some other string solves the issue.
> Also, if I am not mistaken, the files under
> lib/matplotlib/backends/Matplotlib.nib (as returned by the
> get_package_data() method of BackendMacOSX in setupext.py) are needed
> by the cocoaagg backend, not by the MacOSX backend.