Add Currency Conversion

[NSkelsey]

After demoing the app to some folks, it is clear that the Bitcoin only view of transactions is limiting.

The user should be able to toggle between, their local currency and bitcoin price easily.

While the price should not be collected by a centralized API the user could be asked "what the price of the Bitcoin is worth to them." With a warning about possible future changes in price.

[NSkelsey]

This should really be a Character estimate as well. So, the app should tell the user roughly how many characters they can expect to send with the bitcoin that they have.

It should use a fee price based on satoshi per byte of the last mined TX the app sent. Otherwise, it should rely on a sane default hard coded from a service like cointapes.