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

Fixes #875.

I deprecated `RACBehaviorSubject` here because it doesn't really correspond to any other specific 3.0 work, and I wanted to test RACDeprecated.h.

As noted in the code, RAC consumers can declare `WE_PROMISE_TO_MIGRATE_TO_REACTIVECOCOA_3_0` to suppress deprecation messages.