Merge pull request #23 from sixten/develop

Use KVO in a context-aware fashion.
change the key for unscaled images
Merge commit 'eaa99c618b428491ca7d76650782506963f55000' into develop
clear newly-created vImage buffers before processing
Merge pull request #26 from origamilabs/issue-25

[FIX #25]
Updated README for 0.0.3
Updated for 0.0.4 release.
updates podspec for 0.0.4 release
Removes commented-out code.
Merge pull request #27 from origamilabs/0.0.4-release

0.0.4 release
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.