Support addition/subtraction of Money values

I noticed when working with `clojurewerkz.money.amounts/add` and `clojurewerkz.money.amounts/minus` that the functions over-zealously type-hinted as `^double`, when in fact Joda Money's `Money/plus` and `Money/minus` methods accept `Money` and `Iterable<Money>` values.

I've added tests to affirm this behavior and adjusted the implementation to pass those tests.