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.
Can the dosingType be null?  If not, we don't even need the custom display logic on lines 191-200.