DECam utilities

The biggest piece right now is a simplified version of A. Patej's
nightlystrategy.py, which makes observing plans for nights at DECam.

Requires pyephem to run, and can be invoked via: 

python nightstrat.py 2016-03-13 decaps-tiles.fits gr plan 

The above makes a plan for the night of 2016-03-13 using the tile centers in
decaps-tiles.fits, for the gr filters, and outputs the results to plan.json
and sanity plots to plan.png.