Using `.ui-slider-handle` was too broad.

Victor/create docs

Coding standards document

Added django_nose and nosexcover to requirements.txt

Capa cleanup

Fix histogram and sequence navigation bug

looks good, works in my tests. thanks.

This will leave out the student that never attempt the question and
fixes the error with histogram rendering.

There might be a chance that an error got thrown after 'contentChanged'
event was fired and stop the `toggleArrow()` method to be called. This
will make sure that the navigation will still be working even some
element in the content is failed.

PROJECT_ROOT is needed for coffeescript paths

Create dev env

import path not consistent with dev machines?)