Merge pull request #27 from origamilabs/0.0.4-release

0.0.4 release
standardizes on using `NSURL` for files throughout `OGImageCache`

Leaving `filePathForKey:` for backwards-compatibility. (Re-implemented in terms of `fileURLForKey:`
Merge branch 'date-based-cache-purge' of github.com:origamilabs/OGImage into date-based-cache-purge
Merge remote-tracking branch 'origin' into date-based-cache-purge