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.
I'm pretty sure these can't be null. (I see your comment above that these null checks were already here.) I'll probably go ahead and just merge this now, but I expect we'll want to tweak this a bit more before we release and deploy.

(Also, since getDosingInstructionsInstance() is possibly slightly expensive, we should avoid calling it twice.)