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
Add a `requirements.txt` file for one-step install

This allows users who want to install all Python package dependencies in
a single step to do so simply with `pip install -r requirements.txt`.
Of course, anyone without `pip` or who wants to install each dependency
manually is still able to do so by simply ignoring the file.

Note that this commit assumes that the `astrolib.coords` package that is
now on PyPI is the same package that `astrolib/convert.py` intends to
test, even though that code previously seems to have been able to simply
import the package name `coords` without needing to prefix it.  This
might indicate that I have gotten two similarly-named packages confused
and need to adjust that dependency and restore the original form of the
import statement.
Merge pull request #24 from brandon-rhodes/master

Run all tools by default if no “--tools” is given?
Add the Naval Observatory high-precision NOVAS lib
Add 4x speed improvement by avoiding redundant I/O

Generating the simple summary page was taking 10+ seconds on my laptop.
By remembering which coordinate files have already been read instead of
re-reading them, this version runs in about two and a half seconds,
which will make further improvements to the report much less painful to
experiment with.
Add a grid for each tool setting it vs. all others

This lets the author of any one of the tools in question get, in a
single place, a summary of which of the other tools (if any!) their
results agree with.