[FEATURE] Overview: Balance pie RT-1697 #deliver

On the "Balance" page, display the total value of the user's balances, as measured in XRP or any currency for which the user has a trust line and which has a well-defined value according to the RippleCharts "exchange_rates" API. Also, display a pie chart indicating the share of value each balance comprises.
Please use the style guide (src/less/ripple/variables.less) and avoid using hex values so we can keep the client clean with consistent colors.  Looks like you're going for a "@midgray" here.