HTML-759 Rewrite drugOrder tag for OpenMRS 2.x (#231)
This commit pulls the majority of the logic that was built into the drugOrder tag and associated classes, in order to support their use for ordering more broadly, rather than just for drug orders. Most changes in this PR are naming related - changing class names, variable names, message property codes, and adding tags that use generic "Order" terminology rather than specific "DrugOrder" terminology.
