Currency format improvements

What's changed:
* "1,000 bits" instead of "1000 µ" (adds commas with .toLocaleString())
* Fiat now displays as "$600.00" - originally "600.00 $".

Fiat money displays using toLocaleString, this takes into consideration multiple languages when the locale changes ("hi-IN", "en-US", etc).
Fix BTC rounding (from last pull request)

Bug fix for the https://github.com/darkwallet/darkwallet/pull/137 pull.
Merge pull request #137 from thiswillbeyourusername/patch-1

Currency format improvements