Pin WP-CLI to v0.12.0

Keeping WP-CLI on v0.12.0 so we always know what version we're on. We
can bump it manually in the future and fix  stuff that may break. Fixes
#13
Fix Vagrantfile

* There was some brokenness when we set the hostname.
* Maps the MySQL port locally
Fix box name
Make jetpack inclusion explicit

We already had jetpack included via the Developer plugin, but
let's make that explicit here.

Fixes #33
Don't upgrade all installed packages

The ones we care about will be up-to-date because we're installing
the latest versions. This makes the intial Puppet run much faster.

Related #34
Add an overview and Vagrant Primer to the README

Fixes #35
Install postfix so email works
Merge branch 'master' into windows

Conflicts:
	Vagrantfile