Added bitcoin price to status panel (appended to balance)

For example, " 1 BTC~734.55 USD". Uses settings from Exchange Rate plugin and behaves same way as existing fiat balance in status panel.
![electrumstatus](https://cloud.githubusercontent.com/assets/6622335/2539224/0c8b14e8-b5c6-11e3-9f18-0cf55cad3ea4.jpg)