Abstract cache driver for possibility of using other cache backends

[colinmollenhour]

It is good to see work being done on a full page cache for CE, but while you are at it, perhaps it could be abstracted to support implementation of other drivers such as file passthrough (via php engine, great for dev environments), nginx, etc.. Besides, all of the curl methods and such would be tucked away neatly in the driver instead of repeated throughout.

If you don't have time or don't want to do this yourself, are at least willing to accept a pull request for this?