Added tools for Fermi/GBM data, and added GBM as a lightcurve source

Added basic support for Fermi/GBM as a lightcurve source, and some additional tools in sunpy/instr for Fermi. In summary:

  - Added support for downloading Fermi/GBM quicklook (for now binning is fixed into ~7 channels) data for NaI detectors. BGO is not included yet.
  - Added tools to calculate NaI detector angles vs Sun at any time or for a timerange. This info is useful for choosing which detector to download data for.

This is still work in progress. Todo:
  - more cleanup
  - add tests
  - deal with long timeranges in detector angle queries.

It would be possible to add support for arbitrary binning of the GBM data here (the native binning is in 128 channels), but this is a grey area between LC and Spectrum, so it's not clear to me how to proceed there. I'll raise this as a separate issue.




Is this always supposed to always be an integer?  If so, for Python 3 compatibility it should either be case or `from __future__ import division` and `//` should be used, unless time_diff.days can _always_ be guaranteed to be a multiple of 7.