Result of parsing equation is dependent on order of inputs

[zvxryb]

Input	Result
CQ('ax*bx + ay*by=0')	-(ax*bx + ay*by)
CQ('0=ax*bx + ay*by')	ax*bx + ay*by

These are both valid results, but they appear as different expressions which makes unit testing difficult.

[bshlgrs]

We should probably implement a .equals method on nodes, which checks if their simplified and expanded forms are the same.

[MatthewJA]

@zvxryb - This should be a pretty easy fix; I'll put it in the next patch.
@bshlgrs - #112