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.
It would be good to follow the same docstring conventions as Astropy, so call this section:

```
Parameters
----------
```

Then include the arguments as:

```
name : type
    description
```