Lacosmicx

I have finished the optimized LA Cosmic implementation. It uses Cython for the main part of the algorithm and uses C for the individual routines. Everything is multithreaded using OpenMP (the Cython standard). For a 4K x 2K image, this implementation is ~35 times faster than cosmics.py written by Malte Tewes (which is already significantly faster than the original IRAF version). 

Given the complexity of this code, should this be included in imageutils or should it be its own package? 

Any other comments are welcome.
As @astrofrog mentioned before, this is not the correct Astropy / Numpy docstring standard.
Running `python setup.py build_sphinx` should give you warnings for such formatting things, i.e. is a useful way to check the docs (in addition to looking at the generated HTML output via `open docs/_build/html/index.html`).