Negative coefficient of variable does not parse

[calvinw]

In 0.2.4 I have noticed this:

var algebra = require("algebra.js")

This works:
algebra.parse("(-2)_x+3_y=4");

but this does not:
algebra.parse("-2_x+3_y=4");
SyntaxError: Unbalanced Parenthesis

This is a great library...! I am using it and enjoying it tons! Thanks so much for creating this.

[calvinw]

Also with stars (*) for multiplication

-2x + 3y = 4 does not parse

but

(-2)x + 3y = 4 does parse

[timrach]

Hey, I fixed this bug in #42.