Multicast `rawExecutionSignals` for performance
Fix executing/errors not always arriving on main thread

This is done by -deliverOn:ing the values in each signal, though I also
refactored that specific logic into a couple of static functions.
Update documentation (RACCommand isn't a signal)
Refactor RACCommand to deliver normally
Add the originating RACCommand to error.userInfo

This is useful when -execute: is used deep within a signal chain, and
more convenient than using -catch: to add that information.
Restore multicasting of newActiveExecutionSignals
Fix synchronous expectation for [control isEnabled]
omg caps i'm so ashamed
Slightly better example name
Don't save immediateExecuting into a property
Add comment about receiving errors after executions
Document when RACUnderlyingCommandErrorKey is present
Merge pull request #749 from ReactiveCocoa/multicast-command-executions

Multicast newActiveExecutionsSignal, remove re-entrant delivery