result of ARC conversion
change target deployment to iOS 5.0
more ARC changes
README doc update for ARC
added xcuserdata to .gitignore, added project.xcworkspace
fix to operation cancellation

* if an NSOperation is cancelled before it is
  started, NSOperationQueue will still call
  -start and expects the operation to not be
  finished yet. This avoids the queue complaining
  "isFinished = YES without being started by the
  queue it is in".
convenience methods to notify a deferred of the
result of another deferred without affecting the
result of the other deferred
added chainedTo_ ivar
added kHLDeferredNoResult constant
renamed _startRun and _run as per Twisted.

renamed _startRun _startRunCallbacks and _run
_runCallbacks so they match the corresponding
method names in defer.py. This just makes it
easier for me while I'm looking at the two side
by side.
renamed runnning_ runningCallbacks_ as per Twisted
added HLContinuationLink : HLLink
_runCallbacks is now iterative

Previously, _runCallbacks was recursive when
HLDeferreds were chained. Now, it's iterative,
which avoids stack overflows if you have very
deep/long chains.
reimplementing notify: using HLContinuationLink
added -debugDescription method
removed pause and unpause
moved debugDescription next to dealloc
create exceptions in private methods
added private mutator for called_, use it.
fixed notify: