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. 
It should be simpler than OrderType.

Basically, *before* we used to have the idea of DosingType that was an enum, and DosingInstructions that was a class which was 1:1 with the enum.

*Now* we want to simplify, to only have the idea of the DosingInstructions interface, and not have any separate enum or static constant string names. A DrugOrder should have a "dosingType" property whose value in the database is the fully qualified java classname of the DosingInstructions subclass that should be used to interpret that DrugOrder. I think it's an implementation detail whether the DrugOrder.dosingType field has type String or Class<? extends DosingInstructions> (I lean towards Class)