Fixed build error in Xcode 6 beta

Xcode 6 doesn’t allow overriding property access, and extending standard
count property doesn’t work anymore.
Merge pull request #50 from coverback/xcode_6

Xcode 6 support and podspec fix
Made gif serializer strict and only supporting gif

Standard AFNetworking serializer not only assigns image, but also checks
for status codes, MIME types, and inflated images on background thread
before assigning. Loosing this behaviour on all images is unwanted, but
calling super would inflate gifs and cause which could interfere with
OLImage logic.