* Rework cache key generation a bit.
Cache keys are now based on the previous commands, rather than the previous state
of the filesystem.
* Refactor command interface a bit, only cache the context for commands that use it.
CacheCommand returns true if the command should be cached. Currently,
it's only true for RUN but can be added to ADD/COPY later on (these are
different since the contents of files for ADD/COPY need to be included
in the cache key generation).
I also changed CreatedBy to String so that we can log each command
before cache extraction or regular execution takes place.
Cole Wippern
dlorenc
Priya Wadhwa
Nick Kubala
Jon Johnson
Christian Jantz