Flesh out the content on the welcome page.

This is not the final design and content of the Welcome page. It is an attempt
to make it more useful by adding in relevant content that may be useful to new
users and those viewing the application.

- Add three columns of content, for explore, learn and share.
- Add Sign Up button to the welcome page.
Merge pull request #290 from opscode/welcome_page_updates

Add content to the Welcome page