move type conversion and date format handling out of contribs into core.

includes...
use StreamsJacksonMapper with channel-specific date support
deprecate StreamsTwitterMapper
deprecate StreamsDatasiftMapper
use generic TypeConverterProcessor
deprecate/delete now unnecessary classes
refactor event classification
add event classification tests
refactor channel serialization
update tests


InteractionActivitySerializer isn't critical, it's a fallback for documents whose channel is unresolvable.  But I agree it should be tested and will add a test that forces it to run on documents from multiple channels.