Updated to Xcode 5
Formatted the header file
Replaced usage of GCD by NSURLConnection for async connections after a talk with Florian (@dkduck) at Macoun 2013 and removed duplicate implementations of image methods

@elchbenny, you were right! @dkduck told me that using GCD for this purpose is not the best way because it's doing busy waiting on the background queue and thus it can possibly block other operations performed in the GCD queue pool. NSURLConnection's async method, on the other hand, doesn't use busy waiting but the NSRunLoop. It receives callbacks each time some data was downloaded for the request and thus doesn't block another operations.

I've learnt something new, thanks @dkduck!
Bumped version to 0.3