Enforced presence of a completedBlock in downloadWithURL:options:progress:completed:

This method bails out in case of a missing `completedBlock`. While this makes sense (downloading the image without a completion block and not doing anything with it is pointless), a client passing a empty block is not informed about the mistake.

`NSParameterAssert` validates the input and it informs the client about bad usage of the API.
	Updated doc marking the completedBlock parameter of downloadWithURL:options:progress:completed as required
Merge pull request #487 from Gabro/patch-1

Enforced presence of a completedBlock in downloadWithURL:options:progress:completed
Added and exposed some methods that allow to check if an image exists on disk without taking it off disk and decompressing it first. Makes for a nice performance boost when checking if many files exist in the disk cache already
Fixed styling to match rs/SDWebImage repo
Share schema so it can be used by Xcode bots
Only use the runloop with timeout on iOS 5 (fix #497)
Remove WebP support from default target and add a dedicated target

Too many people have issue with WebP library dependancy.
Merge pull request #498 from donholly/master

Added and exposed some methods that allow to check if an image exists on...
Removed strong reference to UIImageView which was causing a crash in the nested block.
Merge branch '63d60d9'
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
update upstream