Form code to clean imported fields

* Empty phone number used to be represented by 0. Now it's NULL.
* Make the city field title-case at import
Import only recent changes
Added new management command to do incremental imports only

* Much faster imports from Durham County due to incremental nature of the data
* Rewrote some of the API code for better clarity
Added celery task to run incremental updates
Model changes: pretty name + inspector's details