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. 
I would probably call this getDosingInstructionsInstance(), and I don't think it should throw any checked exceptions. I.e. catch these and throw an IllegalStateException wrapping them.