Show Current Fiat Price in Status Panel

Cleaned up Exchange Rate plugin: more efficient code and more intuitive names for the get fiat balance/price procedures.  Created new procedure used for updating the fiat balance & price on the status panel.  Enhancement: status panel on bottom left now shows the current BTC price (eg, 1 BTC~573.45 USD).
![electrumstatus](https://cloud.githubusercontent.com/assets/6622335/2543127/1f73db0c-b5ef-11e3-873d-f7ad98367a84.jpg)