Add -calculateSizeWithCompletionBlock:.

This method provides a way to asynchronously calculate the size of the
disk cache, reporting both the number of files and the total file
size.

This is useful when querying large disk caches because the file system
enumeration operation can take some time.
Merge pull request #434 from jparise/calculate-size

Add -calculateSizeWithCompletionBlock:.
Remove an unused call to -callbacksForURL:.

We don't currently signal cancellation via a callback.
Merge pull request #436 from jparise/unused-callbacksForURL

Remove an unused call to -callbacksForURL:.
Merge branch 'master' of https://github.com/rs/SDWebImage into barz-master