feat(deisctl): add deisctl command

This is the beginning of an ongoing effort to create a `deisctl`
command for deis clusters. At the moment, it only runs `help` and
`version`, but the groundwork is there to add more commands to the
client.

Note that this client's architecture is heavily influenced by
https://github.com/coreos/fleet/tree/master/fleetctl.

This PR is also based off of #1034, so that one will need to be merged first before considering this one. See https://github.com/deis/deis/compare/go-test...deisctl for the changes.