0.4.0

• Added a sub type of Money which is named Rate, which mostly works the same way but doesn't support currencies. In many applications monetary values can be multiplied with rates (the rate can for example be the units of an item on an invoice, where the sum of the item row would be the item price multiplied with the rate). The Rate sub type is merely for differentiating monetary values (which fully supports currencies) from the simpler rate type.
• Updated type hinting and object creation methods to work with inheritance.