Using DosingInstructions.getDosingInstructionsAsString when creating a DrugOrder display. Include a display with DrugOrder representations.

This is to include dosing instructions in the standard display of a drug order.  Discussed with @djazayeri for a use case in a pharmacy system being built at github.com/openmrs/openmrs-module-ebolaexample.

I looked at other Resource commits to base my unit tests, let me know if there's a different approach I should've taken.  Also, I left in a lot of the null checks that were already in the display method, didn't fully investigate if they were still necessary.
DrugOrderValidator.java:96 (in openmrs-core) says that dosingType can't be null on an active order. Though I see now that it _can_ be null on a discontinue order. So I guess you do need this check.

Thought this reminds me that if order.action is discontinue, then we should put "Discontinue " at the start of the string we return.