Currency Conversion

This is a continuation of https://github.com/duckduckgo/zeroclickinfo-spice/pull/983.

Checklist:

- [x] Desktop direct-conversion look.

![screen shot 2014-10-14 at 11 17 25 am](https://cloud.githubusercontent.com/assets/81969/4631394/3f01bcdc-53b5-11e4-9a5c-bfecb043cfb2.png)

- [x] Desktop direct-conversion look (dark).

![screen shot 2014-10-14 at 11 26 27 am](https://cloud.githubusercontent.com/assets/81969/4631568/87874d86-53b6-11e4-8060-7596c5158011.png)

- [x] Mobile direct-conversion look.

![screen shot 2014-10-15 at 5 50 46 pm](https://cloud.githubusercontent.com/assets/81969/4654272/9915f53e-54b5-11e4-99e0-32f7511d0831.png)

- [x] Mobile direct-conversion look (dark).

![screen shot 2014-10-15 at 5 52 19 pm](https://cloud.githubusercontent.com/assets/81969/4654275/9e081e00-54b5-11e4-8696-e68c092d3cea.png)

- [x] Make the design work on huge numbers.

![screen shot 2014-10-15 at 5 55 24 pm](https://cloud.githubusercontent.com/assets/81969/4654322/0df22350-54b6-11e4-9a4a-b1951267c5a6.png)

- [x] Trigger on commas (see https://github.com/duckduckgo/zeroclickinfo-spice/pull/983#issuecomment-58547298)
- [x] Don't trigger on "usd to usd"
- [x] Test on small numbers
- [x] Check flags.