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)

I went down this approach, storing the currency as a column in the database, but then decided it was too heavyweight. The encapsulate_as_money works fine under certain assumptions but I didn't think it was a good fit here due to the currency being defined on the Account object, rather than in the line itself. I think one of the first commits in this pr did this.