Add currency support

* adds the option to set the default currency
* adds per-account currency settings
* all balances are in the defined currency for the account
* multiple currencies are supported

![image](https://cloud.githubusercontent.com/assets/11939/3793394/2b793d5e-1b96-11e4-8020-5414318dda2e.png)

It's the default currency for DoubleEntry. Using Money.default_currency means the default currency will change everywhere the money gem is used. 

We could require a currency setting when configuring an account, that way a default is not needed, or maybe use Money.default_currency when none is supplied.