dev-python/pywcsgrid2: Add ebuild for git version of pywcsgrid2

This is a small ebuild for a package I use regularly: pywcsgrid2.

It solves basically the same problems as aplpy in the tree, but using a different approach (aplpy has its own API on top of matplotlib, while pywcsgrid2 implements another Axes class and leaves everything else to matplotlib's API).

The current release depends on pyfits, while the latest git works with astropy and pyfits, so I've been using the latter.