Add a bundled installer for the CLI

This PR adds scripts to create a self contained installer.
This works on linux, windows, and mac and provides all of the
dependencies required to run the CLI.

## How this works

To create a bundle run `scripts/make-bundle`.  This will create
a directory you can zip up and use to install the CLI.

In this directory there will be a single script called `install`.
You can run this script with no args and it will install the CLI.
You can specify things like the install dir and the name of a symlink
to create if you want the `aws` executable somewhere on your path
(like `/usr/local/bin/aws`).

Internally, this is all powered with virtualenv + pip.  Note that
virtualenv is bundled with the installer so you don't need to have anything
pre installed except for python.

When you run the `./install` command, it will create a virtualenv in
`~/.local/lib/aws` and install all the deps under that directory with
pip.

Benefits include:

* Only python is required.
* Does not require sudo to install (though if you want a symlink in
  `/usr/local/bin` you will need permissions to write to that directory).
* Self contained, to uninstall `rm -rf ~/.local/lib/aws`.
* Easy to manage multiple installations of the CLI if necessary.
* All deps are bundled, doesn't require network access to install (works
  if pypi is down).