`jekyll new`: scaffold site generator

This PR adds a new subcommand, `new`, which creates a scaffold site in a specified path, based on the design of @mojombo's site. When I met with Tom, he expressed the wish to have a command like this create a "substantial" site - not just a couple directories and a blank index file. The point of these files is to provide assistance to a newcomer. There's been a lot of talk about being newbie-friendly, and this PR hopes to help with that. From no-jekyll to full-jekyll, a newcomer would just execute:

```
~/code$ gem install jekyll
~/code$ jekyll new blog && cd blog
~/code/blog$ jekyll serve # Navigate to http://localhost:4000
                          # to view your new site!
```

Let me know if there are more files that should be provided with the basic scaffold or if content should be added/reworded in or removed from these files to provide a better scaffold for users.

To-Do:

- [ ] Add tests for `NewCommand`
- [ ] Add page to `site_template` to show that it's not all about posts

Considering:

- `--blank` or `--bare` option, which would just throw blank files and empty directories in the specified directory instead of providing the helpful content
- Adding paths explicitly to `_config.yml` so there's no confusion about what's what (`source`, `destination`, etc)