Feature/126 limit api importer

This fixes #126.

~~It should be only merged 24 hours after #127 is merged, as I would like for a complete import to happen first (that's how I tested it). In theory it should work on any database state... but that's untested.~~

This is still not optimal - the inspections import has a for loop that causes a few hundreds calls to the Durham County API - most of them are unnecessary. Ideas?