initialise kPNGSignatureData data

in SDImageCache initWithNamespace method.
This ensures the kPNGSignatureData is always initialised even if the we
alloc SDImageCache without using the sharedImageCache singleton
Merge pull request #981 from mythodeia/master

initialise kPNGSignatureData data
Update README.md

Picky spelling fix.
Merge pull request #991 from sammcd/patch-1

Update README.md
duplicate failed urls are added into failedURLs

fix bug when download image with option SDWebImageRetryFailed
Merge pull request #994 from initlifeinc/master

duplicate failed urls are added into failedURLs