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
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.
Start of redefinition of constants to allow a 'system' which, if defined, currently just makes that constant an EM constant and requires it to specify a system if it's to be used as a quantity. Not fully working yet though, and the concept is not unique to EM constants either so the concept might be further refined and generalized.
I think these should be the other way around.
The majority of the work of moving the constant definitions back into the
astropy.constants module, reworking the imports so that the circularity
between astropy.units and astropy.constants is overcome, and reworking the
Constant class to have a registry of constants in different systems, among
other differences.
Got the constants module docstring table working using the default unit formatting that was previously used in the docs
Override the si and cgs properties from Quantity to return the Constant defined in that system if it exists, else convert to a Quantity in the appropriate new units.
Use short-circuiting instead of relying on the get() default value to avoid a potentially unnecessary slow unit conversion
Fix the physically incompatible units error to not display None as a sample system
Fixed issues mentioned in #745. Fixed the bug that caused an UnboundLocalError when using e in an expression more than once. Cleaned up no longer relevant comments and docstrings, and updated the documentation.
Added the requested docstring on ConstantMeta (which still isn't great but it's a start) and fixed the mentioned error message for Quantity.__sub__
PEP8 because might as well while we're at it
Fix syntax error (didn't realize parentheses aren't allowed in assignment expressions)
Adds a couple more EM units (I didn't go crazy adding statV and abOhm and the like--if someone really wants those they can either ask for them or derive them themselves--I don't know how commonly these are used?)
Merge pull request #745 from iguananaut/constants-refactor-rebase

Alternate constants API 2