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
Add quotes to header search paths
Clarify -deliverOn: and -subscribeOn:

Specifically, promote -deliverOn: over -subscribeOn:, since the latter
is rarely right.
Merge pull request #541 from lsavino/patch-1

Quote header search path in README
Merge pull request #543 from ReactiveCocoa/better-subscribe-on-docs

Clarify -deliverOn: and -subscribeOn:
Only do work in -rac_description in DEBUG builds
Avoid -rac_description within -description

Since it now doesn't do anything in Debug builds.
Refactor RACCompoundDisposable to use CFArray

Results in a significant performance gain when lots of compound
disposables are in use (and especially nested).
Replace @onExit with try/finally to avoid blocks overhead
Pull disposable arrays out onto the stack for enumeration
Meh, we don't really need exception safety here
Optimize KVO trampoline storage
Merge pull request #549 from ReactiveCocoa/skip-rac-description-in-debug

Only run -rac_description in DEBUG builds