Merge pull request #4 from cdeil/specification

Add full set of celestial conversions; add some documentation
Finish specification table and add notes from @dsberry
Fix rst formatting in Specification.rst
Updated astropy script to reflect latest pull request - now no longer need explicit precession
Updated Astropy script and results to latest version
Merge pull request #15 from astrofrog/updated-astropy

Updated astropy script and results
Big bugfix: report differences in arcsec, not deg
Ran convert.py for astropy, getting slightly different results than astrofrog.
Now all results in the summary files are in arcsec
Merge pull request #16 from cdeil/results_in_arcsec

Big bugfix: report differences in arcsec, not deg
Fixed bug in SLALIB tests
Fixed Astropy script
Added updated Astropy results after e-terms
Updated Astropy results to reflect latest master version
Merge pull request #19 from astrofrog/astropy-with-eterms

Updated Astropy results with astropy/astropy#551
Add NOVAS to Tools.rst
Add SOFA to Tools.rst
Add license info for pyephem

This info was given by Brandon Rhodes here:
https://github.com/astropy/coordinates-benchmark/pull/4#issuecomment-12636741
change astropy convert to match new singular unit naming convention
Run all tools by default if no --tools is given