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.
We should omit the parenthetical expression altogether or make it consistent with the one above it:
> For clusters on other platforms (Rackspace, EC2, DigitalOcean, Google Compute Engine or bare-metal), see...