TRUNK-4445 Added support for custom dosing type for DrugOrders. Changed ...

...the current dosing type enum to a string, while keeping the support for SIMPLE and FREE_TEXT dosing types using constants.
DrugOrderValidator also now takes care of checking for SIMPLE, FREETEXT and null dosing types, any custom dosing types would require their own validation. 
The point of the @Verifies is that it refers to a corresponding @should in the class being tested. I don't see the corresponding @should for this. But let's rephrase this case to say "should apply validation for a custom dosing type" (i.e. requiring brandName is purely a test example, it's not something we want to be documenting)