Precision And Rounding Unit Tests For Money Class

Hi guys,

I've added some precision and rounding unit tests for the `Money` class. The tests are very straightforward, and have definitely helped me better understand the underlying rounding mechanism. I notice that the constructors are accepting negative values as their arguments (tests in `org.broadleafcommerce.common.money.MoneyTest_NegativeAmount.java`). Not sure if this is something that needs to be fixed.