New settings property for task currencies

We introduced the new setting io.ox/tasks/currencies.
This setting contains an array of currency Strings.

The default is ['CAD', 'CHF', 'DKK', 'EUR', 'GBP', 'JPY', 'PLN', 'RMB', 'RUB', 'SEK', 'USD']

This array uses the currency codes from http://en.wikipedia.org/wiki/Currency_code