[hab] Add `hab studio` subcommand.
[hab] Add `hab studio` subcommand.

This change introduces a new top-level subcommand noun: `studio`. The
implementation logic is virtually identical to the `hab sup` subcommand,
meaning that the `core/hab-studio` package is installed if not present
and then directly invoked via an `exec(1)` syscall.

```sh
> hab
hab 0.5.0

Authors: The Habitat Maintainers <humans@habitat.sh>

"A Habitat is the natural environment for your services" - Alan Turing

USAGE:
    hab [FLAGS] [SUBCOMMAND]

FLAGS:
    -h, --help       Prints help information
    -V, --version    Prints version information

SUBCOMMANDS:
    artifact    Commands relating to Habitat artifacts
    config      Commands relating to Habitat runtime config
    file        Commands relating to Habitat files
    help        Prints this message or the help of the given subcommand(s)
    origin      Commands relating to Habitat origin keys
    pkg         Commands relating to Habitat packages
    ring        Commands relating to Habitat rings
    service     Commands relating to Habitat services
    studio      Commands relating to Habitat Studios
    sup         Commands relating to the Habitat Supervisor
    user        Commands relating to Habitat users
ALIASES:
    apply       Alias for: 'config apply'
    install     Alias for: 'pkg install'
    start       Alias for: 'sup start'

> hab studio -t busybox enter
∵ Package for core/hab-studio not found, installing
» Installing core/hab-studio
↓ Downloading core/hab-studio/0.5.0/20160524185743
    2.52 MB / 2.52 MB / [==================================================================================================================================================================================================] 100.00 % 49.32 MB/s  ✓ Installed core/hab-studio/0.5.0/20160524185743
★ Install of core/hab-studio complete with 1 packages installed.
   hab-studio: Creating Studio at /hab/studios/src (busybox)
   hab-studio: Entering Studio at /hab/studios/src (busybox)

[#][busybox:/src:0]# exit

> hab stu rm
   hab-studio: Destroying Studio at /hab/studios/src (busybox)
```

Signed-off-by: Fletcher Nichol <fnichol@nichol.ca>

Pull request: #567
Approved by: reset

rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat