fix duplicate symbols when using with webp via pods
No need for explicit ivar and getters, as long as we have the @synthesize in place
#625 In order to fix the deadlock, reviewed the `[SDImageCache diskImageExistsWithKey:]` method. Based on the Apple doc for NSFileManager, using the defaultManager without the dispatch on the ioQueue to avoid the deadlocks. This instance is thread safe. Also created an async variant of this method `[SDImageCache diskImageExistsWithKey:completion:]`
For consistency, added async methods in `SDWebImageManager` `cachedImageExistsForURL:completion:` and `diskImageExistsForURL:completion:`
Replace #699 Fixed race condition in SDWebImageManager if one operation is cancelled, the completion block must not be called, otherwise it might race with a newer completion for the same object

Conflicts:
	SDWebImage/SDWebImageManager.m
Updated podspec version to 3.7.0
Update to #732 to fix misspell (completion instead of completition)
Updated all block params and properties, using defined blocks that have var names (for proper autocomplete). Created a  SDWebImageNoParamsBlock to distinguish easier from other no params blocks. Fixes #810
guarding against image source == NULL
Merge pull request #819 from n13/master

guarding against image source == NULL
Added test for the UIImage+MultiFormat crash
Fixed SDWebImageCombinedOperation cancel crash (#798 #809)
Merge pull request #795 from xlchen/master

fix duplicate symbols when using with webp via pods
Missing mark.
Merge pull request #827 from BB9z/fix/pargma-mark

Missing mark.
Temporary fix for #809, until we can figure out the cause of the crash
Bumped podspec version
Fixed documentation spelling.
Merge branch 'branch-misspell'
Merge pull request #829 from davemess/master

Fix documentation spelling
Fixed other misspells from appledoc (additions to #829)