Refactor astropy.constants API
Reworks things so that one still uses 'from astropy.constants import e, and then uses e.si, e.gauss, e.esu (e.gauss and e.esu in this case are equivalent).  e.cgs raises an exception.  e still combines with other quantities using its SI value in Coulombs by default, though we could (and perhaps should) modify it to raise an exception by default if a specific system is not used.
For EM constants, show all values in doctoring table
Fix failing test in Python 3.2