Updated requirements and converted HACKING to rst

Conflicts:
	requirements.txt
Some basics of writing files from requests and checking for oldness

Conflicts:
	requirements.txt
days_old working properly
Cleaned up
Writing string instead of bytes to fix py3 compat
More casting
Comparing byte strings now
Fixing writing bytes
Merge branch 'feature/datalib'
Merge remote-tracking branch 'upstream/master' into keplerian_orbits
Skyfield prefers explicit relative imports
Conflicts:
	TODO.rst
First stab at benchmarking suite.
Document our scheme for determining scalar-ness.
Reinstate doctests; turn off introduction.rst

The examples in introduction.rst not only no longer work, but they no
longer accurately reflect my vision of how simple the API can be now
that Skyfield has started barely working; so I will slowly re-work each
one, bringing each back online as it works.
Fix a "dev.rst" line to work also under Python 3
Default py.test arguments, for easier dev story

These had been commented out, and are hereby reinstated.
Switch "planets.py" to being only planet objects

The actual Ephemeris class is now housed in the jpllib module, where it
can be imported without the expense of loading DE421.
Give the earth object a topos() method

Also, eliminated the need to give it an "earth" attribute when it
already has the whole "ephemeris" attribute of which earth is a part.
Make horizontal() return actual Angle objects
I think i've inutted the formulas correctly, but I'm not getting the right numbers.