feature: Start on a signup page.

* Requires updates everywhere - client side, server side, etc.
Remove a lot of cruft, finish the new user page.
feature(server): Add mongodb backed session storage.

* Extract mongo connection routine into its own module.
* Add session-store.js which connects to the db and returns a MongoStore.
feature(server): Allow routes to return httpErrors which are automatically handled.

* Add httpErrors to allow simpler error handling.
* Add tests for httpErrors and the routes module.
* Fix broken errors.
refactor(test): Split up db_test.js into individual model tests.
feature(server): Add session creation/deletion functionality.
feature(server): Start on the notion of authenticated/authorized users.
feature(server): Users must be authorized to view a site to actually view it.
feature(server): Add a signin/signout button in the header. Add ability to add a site from /site
feature(server): Combine the signin and signup pages. Add session status to all pages.
feature(server): Add generic page authorization handlers.

* Add a handler to each page.
fix(build): Ensure signin.min.js is added to the list of uglified files.
Merge branch 'users'
feature(server): Add a /welcome screen.

* Screen shows user how to add snippet to site to start collecting stats.
Merge branch 'better-signup'