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
Use RandomState in iterative tests
remove a commented line
Add mass-energy conversion.
Add some unit tests for the mass energy equivalence.
change the seed
Remove old compatibility tools from asciitable that are not needed in astropy

The old asciitable worked for python < 2.4 and worked without numpy.
Some of the compatibility stuff for that was still in the io.ascii
codebase, although this situation can never arise in astropy (it requires
numy anyway).
Orignially, it was left there to make it easer to develop io.ascii and
asciitable in parallel, but by now io.ascii is so closely realted to
astropy.table that it is unfeasable anyway to remake a stand-alone package
based on this. Thus, I removed the old code.
Add energy surface density, density, and power to mass_energy.

* Three new equivalencies in mass_energy
* Add unit tests for them
Change spacing around **
Regression test to ensure equivalencies are passed on in `to(...)`
Merge pull request #1359 from mhvk/units-quantity-keep-equivalencies-regression-test

Regression test to ensure equivalencies are passed on in `to(...)` (follow-up on #1356)
Remove any import