Allow user to select an alternative currency and specify amount of money in a fiat currency

Addresses #1187

User can select which currency to use in the settings screen (defaults to USD).

Then, in the send screen, he can type in an amount in fiat and it will update the amount in btc/mbtc/ubtc/sats.

Uses https://bitpay.com/api/rates

![screen shot 2014-08-27 at 5 27 39 pm](https://cloud.githubusercontent.com/assets/42750/4066901/9ccfd6d0-2e28-11e4-94e7-a19266db9a7e.png)
Maybe we should `throw` here, to prevent unexpected results.