Npred cube2

Implementation which I intend to use to produce significance images (currently this is still running on my machine).

The unit tests will need much improvement soon as I just implemented some quick ones. Looking at the outputs with ds9 using the FermiGalacticCenter files seemed reasonable, but obviously not rigorous yet.

Don't know if you were aiming to merge this asap, so did a pull request just in case.
I think we should start using [APLPy](http://aplpy.readthedocs.org/en/latest/) for sky image plotting to have sky coordinate labels.

The Sphinx build on travis-ci as well as the docs build on readthedocs are already configured such that this should work.

There might be some plotting applications that require using [WCSAxes](http://wcsaxes.readthedocs.org/en/latest/) directly instead of using the higher-level `APLPy`.