added currency formatting for labels

This change formats the revenue amounts to a number with a leading $ sign, with commas, and two places fixed precision after the decimal point.

These examples helped me figure out the formatting:

http://bl.ocks.org/zanarmstrong/05c1e95bf7aa16c4768e

http://koaning.s3-website-us-west-2.amazonaws.com/html/d3format.html