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