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.
Add license line and 
```
from __future__ import (absolute_import, division, print_function,
                        unicode_literals)
```
at the top of this file (per the [astropy guideline](http://astropy.readthedocs.org/en/latest/development/codeguide.html#interface-and-dependencies).

Especially the division can change rounding / one-off behaviour, so might be relevant here.