Fix concurrency in NSControl.rac_command

This necessitated giving up Cocoa bindings, since there's no way to
change the thread that those trigger upon.
Fix concurrency in UIBarButtonItem.rac_command
Fix missing import
Document that RACAble… may introduce retain cycles
Add an example of @strongify with RACAble
++formatting
Merge pull request #538 from ReactiveCocoa/fix-command-binding-concurrency

Fix concurrency in provided rac_command properties
Merge pull request #539 from ReactiveCocoa/better-racable-documentation

Documentation warnings about retain cycles with RACAble
Merge branch 'master' into 2.0-development