Allow users to create Tools

:fork_and_knife: 

This is ready for review!

This introduces the `Tool`, a way for folks to share their various tools and plugins on Supermarket. A user can only create a tool in this, and the UI is not at all complete.

https://github.com/opscode/supermarket/commit/68e8e5dfa1db242531b71bff7fefa4b2de6665f4 is a commit that allows for the enable/disable of tools in the UI which allows us to merge into master & deploy w/o having incomplete work on prod.

Trello card: https://trello.com/c/oV2dAj7h The todos lives in the Trello card. :smile_cat: