Add currency support

* adds per-account currency settings
* all balances are in the defined currency for the account
* multiple currencies are supported
* default currency is `Money.default_currency` if not defined on an account. Money.default_currency can also be set using the Money gem.

![image](https://cloud.githubusercontent.com/assets/11939/3793394/2b793d5e-1b96-11e4-8020-5414318dda2e.png)