NF - added signal-to-noise function to stats
Changed function name of SNR to signal_to_noise
changed function name and cleaned up some grammar
added gain parameter and made suggested changes to documentation
removed gain and fixed some errors
Added gain and updated documentation
added simple test for snr function
added more tests for snr function
Merge pull request #870 from astronomeralex/add-snr-to-stats with manual conflict resolution

Conflicts:
	astropy/stats/funcs.py