Revert "Fixed CGBitmapContextCreate warnings for invalid alpha properties"

This reverts commit 12196f4de69268dc994ba3d5bc04a11568177520.
It adds alpha on JPEG with no alpha.
Fix downloader queue using assign instead of strong under iOS 6
Add synchronous disk-cache loading method.
Merge pull request #297 from inamiy/disk-cache

Add synchronous disk-cache loading method.
Merge remote-tracking branch 'upstream/master'

* upstream/master:
  Add synchronous disk-cache loading method.
  Fix downloader queue using assign instead of strong under iOS 6
  Revert "Fixed CGBitmapContextCreate warnings for invalid alpha properties"