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 think you might need a convenience method that returns the dosing type class object, @djazayeri and @bmamlin what do you think. So that when making comparisons like the one below in getComplex(), we compare class objects and not class names which is not always correct since two classes are said to be identical if they are loaded by the same classloader