'wagtail start' command

This pull request adds a new command called "wagtail start" which gets installed into the users environment when they install wagtail.

It also adds a project template which includes:
 - A core app with migrations to replace the default Homepage
 - Base templates (base, 404, 500)
 - Vagrant configuration
 - Fabfile
 - Docs directory
 - Split up settings configuration (different settings for dev and production)
 - Requirements
Presumably this stuff can be taken out now, since no-one will be looking in wagtail/project_template/readme.rst for documentation on the template itself?