Add cascade option to drop tables tool

The script will fail without this as it needs to cascade to other
relations
Add comment explaining add_node params

It's difficult to see what is required without this
Merge pull request #227 from alphagov/explain-add-node-params

Add comment explaining add_node params
Merge pull request #226 from alphagov/add-cascade-option-to-drop-tables-tool

Add cascade option to drop tables tool
Implementation of returning modules

Return only the module if the slug contains a module slug - that is the
slug is of the form /<dashboard-slug>/<module-slug> - ie a dashboard has
been found for <dashboard-slug> and there is a remaining part of the url
that corresponds to a slug in the dashboards modules.
Add logic for finding tabs when specified

Also refactored the module finding logic as some of it was similar to
tab finding logic