Add sign before unit when converting negative numbers

NumbersHelper.number_to_currency(-2.99) should return "-$3", not
"$-3". This is consistent with what rails does.

I also did a rather significant refactor of the function. Let me know
if this is poor CoffeeScript style; I'm very new to the language.
Merge pull request #4 from scpike/handle-negative-numbers

Add sign before unit when converting negative numbers