fix a race when multicasting to a replay subject /ht @jspahrsummers

The replay subscription could be disposed of before it had a chance to
actually do the replay.
back to using a BOOL to track whether we've connected
don't need to synchronize for the disposable anymore
Added -[RACScheduler after:schedule:]
Make -delay: work in the background and delay completion

Only errors should be propagated immediately. The signal should not
complete until all previously-scheduled values have made it through.
Refactor -throttle: to work on background threads
Remove -rac_performBlock:afterDelay:

It's not safe for use on background threads, and RACScheduler has a
much better API for this now.
Slightly less synchronization for -throttle:
Replace +flatten: with +merge: in the README
Explain +merge: a bit more
Added RACNamedSignal() to declare and name a signal
Added RACNamedSequence()
Add -[RACStream setNameWithFormat:]
Remove +createSignal:name: in favor of -setNameWithFormat:
Use -setNameWithFormat: to clean up name-setting code
Added -replayLast
Copy the headers to the new standard location in Xcode 4.3
Merge pull request #246 from github/fix-throttle-and-delay

Fix -throttle: and -delay: for background threads
Merge pull request #248 from github/flatten-replaced

Replace +flatten: with +merge: in the README
Merge pull request #252 from danielctull-forks/master

Copy headers to the standard location in Xcode 4.3