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. 
Is there some reason that we don't have dosingType as a Class<? extends DosingType> or something like that?