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)

Do you mean when we create an account, we set its currency to be the default if none specified?

That could work, but then you'd have to set the default currency before you created the accounts.