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.
The description is not very good ... it should say "code that doesn't use 2to3 should add these __future__ imports".

I don't have time to explain why `__future__` is useful here, but there's a ton or articles / blogs / docs on the web, e.g.:
http://astropy.readthedocs.org/en/latest/development/codeguide.html#welcome-to-the-future
https://docs.python.org/3.4/howto/pyporting.html
http://python-future.org/

The main concern in this file is that an expression like `kernsize / 2` can behave differently in Python 2 and 3 because integer division is different (and numpy indexing with floats is different in different versions).
So it could happen that your code gives different results (i.e. is buggy) in corner cases on Python 2 or 3.
Putting `from __future__ import division` does nothing on Python 3, but makes division behave like Python 3 if you run on Python 2.

OK?