Add get_appropriate_subclass to XFormInstance, dispatched on doc_type
Move class-instantiation dispatch out of XFormInstance
Merge pull request #7 from kennknowles/XFormInstance-db-subclass

Dispatch model class instantiation based on stored doc_type
delete duplicate safe_index
Merge pull request #8 from dimagi/cz

delete duplicate safe_index
Merge branch 'master' into dmyung-dev
fixing xform.js to skip devicereports AND include all variants of XForm submits (dupes, etc) so that the index can index them and we can filter them out on search