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)
That's correct. I think this pattern should be abstracted away and could be useful to, for example, get notified as soon as we have an updated balance.

The timeout is 1 ms because I seem to remember that firefox (or another browser, maybe IE) has an issue with setting up a callback with 0 ms. It's a pattern to avoid a deeper call stack and make the call asynchronous (in node you use process.nextTick()).