Add RACPromise class
Add -[RACSignal promise]
Clarify that work is only guaranteed to run when started
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.
Tests for RACPromise
Merge branch 'deprecations' into promises
Verify startCount after -autostart
Clarify RACMulticastConnection deprecation message
Merge branch '3.0-development' into promises
Rename -[RACPromise autostart] to -deferred