Mark all 2.0 deprecations as unavailable instead

Stop maintaining their implementations. Everyone should have migrated
by the time they're on 3.0.
Add RACDeprecated macro to mark 3.0 deprecations

Consumers can define WE_PROMISE_TO_MIGRATE_TO_REACTIVECOCOA_3_0 to
squelch these warnings/errors.
Deprecate RACBehaviorSubject, test RACDeprecated
Merge branch '3.0-development' into deprecations
Merge branch '3.0-development' into deprecations
Merge pull request #878 from ReactiveCocoa/deprecations

Remove implementations deprecated in 2.0, add RACDeprecated macro, deprecate RACBehaviorSubject