Implement 'use' to download specific ISO.

Note that no garbage collection of old ISOs is needed because this overwrites the only saved ISO.
This addresses issue #306.
so does this work if i type... `docker download 0.2` and `docker download v0.8.4` - or is it specific to some magic format?

also - i personally would prefer that if we're going to download more than 'latest', that we store the iso's using the version number in the name - so its obvious to the user what will happen next, and to avoid constantly downloading the same files.