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)

We don't need to store the config on `DoubleEntry`. It should go somewhere else so all can use it.