PSF/PRF photometry prototype

As part of my GSoC project I've implemented the basic functions to perform PSF/PRF fitting in photutils.
I've tried to follow as close as possible the proposed [photometry API](https://github.com/astropy/astropy-api/pull/5/files). Up to now it is possible to create `DiscretePRF` from data, to fit `DiscretePRF` and  `GaussianPSF` and it is also possible to remove PSF from the data to create residual images. 

I've written an IPython notebook [PSFPhotometry](http://nbviewer.ipython.org/6550989/PSFPhotometrySpitzer.ipynb), which demonstrates the use of all this functions on Spitzer data. 

If you want to test the code yourself (In fact I'd like to invite you to do this...), it may be best just to clone from the [PSFPhotometry Gist](https://gist.github.com/adonath/6550989), which also includes the necessary catalog data.

I'm looking forward to any comments and discussion!