## ChangeDispenserController Class

### Methods
#### calculateChange(unitsTendered, price)

**unitsTendered:** An array of instances of some kind of MonetaryUnit subclass.  The units tendered all have the same currency and that the amount tendered is greater than price.

**price:** Number. You can assert that the units tendered match the currency for price, which is why price is just a number.

**Returns** true if change can succesfully be dispensed, otherwise returns false to issue a refund and cancel the product.

## ChangeDispenser Class
An instance of ChangeDispenser is programmed to physically know what change it has to dispense and physically how to dispense it.  Assume that ChangeDispenser is a thin JavaScript layer over native code.

### Properties
For every currency unit available, the JavaScript layer dynamically defines properties that match the class name of the monetary units using Object.defineProperty. It defines the properties as read-only properties using getter methods that return new arrays.
The new arrays returned from these properties are not referenced internally. For example, `someChangeDispenserInstance.UnitedStatesPennyUnit` would give you an array containing all available instances of UnitedStatesPennyUnit.

### Methods
#### unitsAvailable(currency)
Describes the types of monetary units available to dispense.

**currency:** A String value for the currency, i.e. "USD".

**Returns** an array of objects describing the availablity of monetary units that match the curreny passed in, sorted in descending monetaryValue:
```
[
    // ...
	{
		propertyName: String (i.e. "UnitedStatesDimeUnit"),
		unitInfo: {
			monetaryValue: 0.1,
			currency: "USD",
			currencySymbol: "$"
		}
	},
	// ...
]
```

#### dispense(units)
Physically dispenses change for all of the monetary unit instances that you pass in.

**units:** An array of instances of some kind of MonetaryUnit subclass.

**Throws** an error synchronously if any instance is not in the monetary unit properties, i.e. `someChangeDispenserInstance.UnitedStatesPennyUnit`.