Update UIImageView+WebCache.m

Can cause animation losing or crash if you are in a thread.
Fixed #900 by adding a new flag SD_LOG_NONE that allows silencing the SD logs from the Prefetcher
Merge pull request #896 from skrew/master

Update UIImageView+WebCache.m
Turn of logging
Merge branch 'master' of github.com:dezinezync/SDWebImage