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 'deprecations' into promises
Deprecate RACReplaySubject, fix up users of it
Deprecate +start… in favor of RACPromise
Deprecate RACMulticastConnection

Prefer plain RACSubjects for non-replaying behavior.
Prefer RACPromise for replaying behavior.

This also makes connection semantics more explicit and hopefully less
confusing.
Deprecate multicasting and replaying operators

Use RACSubjects or RACPromises instead.
Refactor -rac_liftSelector: to use a replay subject directly
Refactor NSFileHandle extension with RACPromise
Refactor NSData extension with RACPromise
Refactor NSString extension with RACPromise
Suppress all RAC deprecation warnings in tests
Require a scheduler for RACPromise

This ensures that the subscription and delivery threads are
better-defined and less surprising.