Allow users to create Tools

:construction: :rotating_light: :construction_worker: :rotating_light: :construction:

This is current in progress, and it is kind of a beefy PR.

This introduces the `Tool`, a way for folks to share their various tools and plugins on Supermarket.

Trello card: https://trello.com/c/oV2dAj7h

The todos lives in the Trello card. :smile_cat: 
I'm not sure if you need to check for `current_user` first. I believe `current_user` returns nil if there isn't a user logged in so nil would not equal `@user`. It works [here](https://github.com/opscode/supermarket/blob/tools_create/app/views/users/_sidebar.html.erb#L65).