Remove Vagrantfile and remove it from all docs

This removes the Vagrantfile and updates the documentation to remove the steps which explain how to install Docker in a VM via Vagrant.

This PR also modifies the Windows install guide to use boot2docker.

There are a few reasons for removing the Vagrantfile and the docs which explain how to install Docker via virtual machines using Vagrant:

- Vagrant and VirtualBox need to be downloaded and installed on the machine; Vagrant was an unnecessary dependency for the EC2 example
- The Vagrant boxes aren't provided via HTTPS and they're large downloads. They'd still remain large downloads even if we made our own images available via HTTPS.
- Vagrant relies on virtual box guest additions to set up the virtual machines. This is a problem because the guest additions need to be kept up to date when upgrading VirtualBox.
- The VirtualBox guest additions ISO file isn't available via HTTPS.
- Provisioning a new Vagrant virtual machine takes a long time and Vagrant's VirtualBox VM hangs at the boot prompt on OS X sometimes. This can be fixed, but this doesn't fix the other problems.
- The user has to download Vagrant and VirtualBox. The vagrant box needs to be downloaded as well if the user didn't already have everything set up and didn't use the Vagrant box we require.
- Windows really needs a simple and easy to set up package. The guide required more steps to follow and tools to be set up on Windows.