Features/casa

@garymacindoe 

Adds [CASA](http://casa.nrao.edu/) and [pyrap](http://www.astron.nl/casacore/trunk/pyrap/docs/index.html) wrappers. 

Both CASA and pyrap are quite similar for the purpose of this pull-request. In both cases we need only to access a python wrapper to the underlying casacore Tables. Since both wrappers have similar bindings, we can use them both interchangeably. The main difference is that CASA adopts Fortran indexing conventions whereas pyrap adopts C indexing.

Since we do not have CASA measurements per se, it is difficult to manufacture tests. I have repurposed the [NGC 4286 tutorial](http://www.astro.uni-bonn.de/ARC/casascripts.shtml) in  scripts. The `get_data.py` gets the data and does some treatment. The other two scripts run incomplete purifications on both channels. 
The pyrap script should fail when trying to create an image, as a I havn't figured out how to tell pyrap where the extra data it requires can be found.

- Closes #41 
- Closes #39