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


Can you take these commented lines out if you're not intending on using them?