Create a `leg init` command to initialize a new tutorial

[paigeruten]

Try to make the process of creating a new tutorial from scratch as user-friendly as possible. leg init should initialize a leg project in the current directory, while leg init <project-name> should initialize a leg project with the given name in a new directory of the same name.

What to generate:

<project-name>/
  diff/
  template/
    page.html.erb
    step.html.erb
    style.css.erb
  leg.yml
  .gitignore

[paigeruten]

Moved into #29