Refactor toCurrency outside BitCoin

[makevoid]

https://github.com/720kb/BitNFC/blob/master/www/js/factories/bitcoin.js#L246

it is not related to the wallet

[makevoid]

also decide if we want to rename BitCoin to Wallet

[wouldgo]

Wallet is better, since it's a wallet api by now. Am I right?