update release checklist with docs instructions
opt-in to unknown fields tracking; ignore retired fields

Conflicts:
	src/main/ssp/codegen/scala/class_companion.ssp
	src/test/thrift/com/foursquare/spindle/codegen/spindle_test.thrift
fix type for missingAlternateObj hook
TReadableJSONProtocol: handle null string fields
2.0.0-M3

forward-port fixes from 1.8.4