ASBasicImageDownloader iOS 7 support

On iOS 7, NSURLSessionTask is actually a __NSCFURLSessionTask (private class) and our NSURLSessionTask category to add the associated object asyncdisplaykit_metadata was crashing on iOS 7 stating that there was no method. This commit changes the category to extend NSURLRequest as well as change NSURLSessionTask to NSURLSessionDownloadTask.
Update manual display logic

Postpone manual display until a future release when it can be called on any thread. Provide the current node manual display logic as a category on ASDisplayNode only available for ASRangeController. Deprecate -displayImmediately.
Merge pull request #139 from facebook/imagedownloader-ios7

ASBasicImageDownloader iOS 7 support
Merge pull request #140 from facebook/manual-display-part-deux

Update manual display logic
Typecast objc_msgSend call in tests.
_ASTableViewProxy & _ASCollectionViewProxy fixes.

These weren't correctly handling teardown:  nilling the `asyncDelegate`
created a new selector-forwarding NSProxy that crashed and burned
because there was nowhere to forward to.  Nil everything correctly.

Tentatively fixes #121.
Merge pull request #141 from facebook/proxy-teardown

_ASTableViewProxy & _ASCollectionViewProxy fixes.
Fix header visibility.
Merge pull request #2 from facebook/master

Merge from facebook/master