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
added Msun alias for solar mass unit
Updated the 'Installing Astropy in develop mode' section (use setup.py develop in the first instance, added a link to the instructions on how to run the tests, which required adding a label to install.rst), added some additional links to GitHub pages, fixed some typos, applied other minor edits for clarity / consistency
spectral equivalencies from multiple conventions (work in progress)
relativistic equivalency implemented with example
implemented @astrofrog's comment: [convention]_velocity ->
doppler_[convention]
added doppler equivalencies to __all__
refactor equivalencies: include freq, wav, energy conversions for
doppler
fixed equivalencies issues, but tests won't run
added equivalency tests
added one more test.  I don't think I changed equivalencies...
tests fix
added new spectroscopic equivalency example