Include gulp-connect for development

Run `gulp connect` to get a webserver up and running.

I'm not sure how best to make this work with the watch task. For now,
you can just run both tasks.
Show dashboard slug on dashboard

To show how the dev mode works.
Merge pull request #11 from alphagov/dev-mode

Include gulp-connect for development
Add a simple carousel

This uses CSS 3 transitions and a bit of JavaScript to move
through several slides.

The code was inspired by https://github.com/Demotive/gds-data-screens,
which is a look that people like.