Abstract out cache to different adapters

[listrophy]

Problem:

Currently, the upload cache uses only a file-store for storing the upload. This wouldn't really work in a multi-application-server environment without having to implement session affinity at the load balancer level.

Proposed Solution:

Abstract out the caching layer so that you could swap in any number of adapters (e.g., file, memcached, s3, etc).

[bensie]

Pull requests are welcome! Think you can work up a patch?

[listrophy]

Yeah, already working on it, but I can't promise I'll have the time to finish... :(

[aaronchi]

hmm... this would be awesome for heroku :)