WIP: Introduce deisctl and new cluster-aware development environment

This PR introduces `deisctl` to the project and provides a new cluster-aware development environment based on building and pushing Docker images to a private registry.

 * Removes `fleetctl` as a dependency for Deis usage and operation
 * Deprecates the tangled web of `fleetctl` calls in the `Makefile`
 * Refactors `Makefile` targets to use `deisctl`, adds new `dev-registry` and `push` targets
 * Changes to root project README
    * Switch all references of `fleetctl` to `deisctl`
    * Change instructions to provision a 3-node Vagrant by default
    * Clean and shorten content
 * Updates local development instructions (coming soon)

Note: new local development environment assumes Vagrant and Boot2Docker.  Doc updates for this coming soon.
Do we want to continue to say "Documentation for other platforms is forthcoming?" and the following sentences in this paragraph? Our current subset of CoreOS' platforms is likely to remain the same. I think this isn't needed.