Multi-currency support

[adamcharnock]

Work ongoing on feature/multicurrency, see multicurrency docs.

Adding support for multi currency.

• Add the concept of the 'base currency' (i.e. the main currency for the app)
  • No, let's not do that
• Parent accounts can combine currencies of child accounts to their primary currency. To do this they should use the exchange rate entered for the transaction.
  • Transactions do not have exchange rates. This should use the current exchange rate.
  • We do not intend to have a primary currency as such, but perhaps a default display currency. This default display currency should be easy to change based on the user's needs.
• Need historical source of exchange rates (find an API) (fixer.io)[http://fixer.io]
• Transactions have an exchange rate which can be user specified, but defaults to the exchange rate on the date of the transaction
  • No. Currency exchange is done via trading accounts, transactions are not aware of any exchanges.

Therefore:

• [x] We need a Currency field/class which represents an decimal value plus it's currency (Done: uses py-moneyed)
• [x] Create way of representing an account balance which may have multiple currencies
• [x] Accounts have one or more currencies (except bank accounts which have one, and trading accounts which have at least 2)
• [x] Implement trading accounts

[adamcharnock]

Closing as this feels pretty much complete. Will continue to test during implementation of battlecat & swiftwind.