[nim] #8 Adds distribution plan node object.
[nim] #8 Links distribution plan node to distribution plan. Makes distribution plan node editable in django admin
[nim] #8 {WIP} Some tests commented out. Pending completion of introduction of DistributinPlanNode object
[nim] #8 Can create ditribution root distribution plan node
[nim] #8 Creates distribution plan node with parent
[nim] #8 Can get distribution plan nodes with their parents
[nim] #8 Exposes 'children' as key in node json returned by API
[nim] #8 Extracts API test helpers into their own module to avoid import issues
[nim] #8 Distribution plan now avails all its nodes through API
[nim] #8 Can create distribution plan line item linked to a distribution plan node
[nim] #8 Distribution plan nodes now have their line item set when querried through API