Add and register the debugtoolbar package

This now renders stack traces in the browser rather than the vauge and unhelpful "Internal Server Error" message. This is a godsend when trying to debug issues with SCSS files as I couldn't get stack traces or anything logged to the console when running the development server.

The package is suggested in the Pyramid documentation[1].

Here's how it looks:

![debugtoolbar](https://cloud.githubusercontent.com/assets/47144/3524735/00c45c94-076c-11e4-8054-72065dfd50e2.png)

And before:

![servererror](https://cloud.githubusercontent.com/assets/47144/3524744/2ac3bd1e-076c-11e4-95f9-0dcd9c4a1591.png)

[1]: http://docs.pylonsproject.org/projects/pyramid/en/latest/quick_tour.html#easier-development-with-debugtoolbar