This change upgrades to PINCache 3.0.1-beta which supports
custom archiving and unarchiving.
This allows us to avoid using NSArchiver/Archiver which adds a
small amount of disk space overhead and between .5 and 10 ms (on an iPhone 4)
for each encode / decode.
To support this, we *do* have to add versioning for the cache and
destroy it for the old versions. I considered adding a process to migrate
the images, but because that would have to occur on startup and process
all existing images, it could cause issues for very large image cache
databases, so I decided against that.
