remove redundant `CGImageSoureCopyPropertiesAtIndex`
add date-based purging to `OGImageCache`
adds 'queue-jumping' for file system-related tasks.
standardizes on using `NSURL` for files throughout `OGImageCache`

Leaving `filePathForKey:` for backwards-compatibility. (Re-implemented in terms of `fileURLForKey:`
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
fixes comments
makes potentially long-running purge methods run as background tasks
updates comments to explain background task behavior
fixes bugs in demo app.

- Move purging to `applicationDidEnterBackground`
- Fixes bug with `detailTextLabel` -- now we just show any errors
- Moves `OGImageTableViewCell` to latest observer methods.
fixes comment: WWDC 2011, not 2012
Merge pull request #29 from origamilabs/date-based-cache-purge

Date-based cache purge. Thanks to @sixten for all the feedback.
Minor formatting in README.md
Merge branch 'master' into develop
Merge branch 'develop' into ogimageview

Conflicts:
	OGImageDemo/OGImageDemo/OGImageTableViewCell.m
	OGImageDemo/OGImageDemo/OGViewController.m