Merge pull request #522 from redhand/master

Removed strong reference to UIImageView in nested block
Fix WebP target
Add option to continue download in background
Provide background option through the manager
Merge pull request #534 from pixable/ios-background

iOS Background: Provide background option through the manager
Fix link to "Who Uses SDWebImage" wiki page
Merge pull request #537 from raylillywhite/patch-1

Fix link to "Who Uses SDWebImage" wiki page
Update argument list for example in README
Merge pull request #539 from sibljon/patch-1

Update argument list for example in README
Use UIImagePNGRepresentation to preserve transparency
Only perform `NSLog` statements if `SD_VERBOSE` is defined
Merge pull request #551 from robertmryan/master

SDWebImagePrefetcher should only perform `NSLog` statements if `SD_VERBOSE` is defined
Merge pull request #552 from garnett/master

Use UIImagePNGRepresentation to preserve transparency (fix for #545)