Merge branch 'develop'

* develop:
  Potential strong cycle fixed.
  SDWebImageDownloaderQueueMode type renamed. Fixed typo. Added description for renamed type. Type renamed because "queue" notion is a FIFO only, but LIFO is a stack, and if we give the type a neutral name, we can avoid logical inconsistencies.
  Fixed typo.
Merge pull request #351 from Vyazovoy/master

New name for SDWebImageDownloaderQueueMode type, typo fixing, strong cycle fixing.
-getSize now returns an `unsigned long long`.

This type now matches the type and width of the value returned by the
`fileSize` accessor of the `NSDictionary (NSFileAttributes)` category
(which is what is used when accumulating the total cache size).
Merge pull request #352 from jparise/getSize-return-type

-getSize now returns an `unsigned long long`.
Remove double import for SDWebImageDecoder.h
Merge pull request #355 from u16suzu/fix_double_import

Remove double import for SDWebImageDecoder.h