Properly set HL7Source on generated HL7 queue objects

Currently, the FormEntryQueueProcessor sets a new HL7InQueue's HL7Source property to the same thing every time:

hl7InQueue.setHL7Source(Context.getHL7Service().getHL7Source(1));

This value can be generated in a few different potential ways:

1. An element in the XML data (requires modification to all existing form schemas)
2. A source name on every form by way of Form Resources (only in OpenMRS 1.9+)
3. A default source for all FormEntry forms (will not provide for weeding out particular sources, but makes sense to have regardless of the final solution)
4. By connecting a given source to one or more encounter types (my favorite)