[FIXES #8]

Multiple requests for the same URL will now only result in a single network request. No changes to the `OGImage` and subclasses' interfaces. Changed the interface to `OGImageLoader`
fix thread-safety for delegate array access and delegate calls
fix bug in idempotent test
Merge pull request #12 from origamilabs/bugfix-8

[FIXES #8] Idempotent network requests