Implement more complete JSON serialization for sympy

[mpharrigan]

Follow up to #1880 (review)

It would be ideal if sympy itself had a serialization format. However, the most promising candidate srepr looks to be horribly unsafe to parse on untrusted input.

#1880 adds support for sympy.Symbol, but we'll probably need support for more constructs; at least arithmetic

[dabacon]

More support has been added for simple math. Do we have more we'd like to add? I'd recommend we close until we see if there is more complicated math that starts showing up that we need to serialize.

[mpharrigan]

Sounds good to me!