Major refactoring of API

Closes #15

@devcritter could you review this. I made some major changes to simplify how the content handler works and to better suit what we need for the XDS.b module. If you can't review both the PRs I just submitted, reviewing this one would be most helpful.
Cool. Maybe it'd be useful to use the contentId for comparing Content objects here: https://github.com/jembi/openmrs-module-shr-contenthandler/blob/api-refactor/api/src/main/java/org/openmrs/module/shr/contenthandler/api/Content.java#L229
i.e. `return contentId.compareTo(o.contentId)` instead of payload?

I was never happy about using payload there, so you gave us a nice alternative now :)