Allow user to reset experiment (closes #64)

beat/web/experiments/api.py

beat/web/experiments/static/experiments/css/style.css

beat/web/experiments/templates/experiments/experiment.html

beat/web/experiments/templates/experiments/setup.html

beat/web/experiments/templatetags/experiment_tags.py

beat/web/navigation/templates/navigation/user.html

beat/web/ui/static/ui/css/contribution-reset.css

beat/web/ui/static/ui/js/list.js

beat/web/ui/static/ui/js/reset.js

beat/web/ui/templates/ui/contribution_reset.html