User specs for validity

This branch includes:
* a fully-fleshed out User spec, which checks valid additions as well as invalid ones
  * at the terminal, run `cd icookie` then `rspec spec --color`
* a rake task for populating the dev database with all the info from the zip code CSV
  * at the terminal, run `cd icookie` then `rake import_locations_csv:create_locations`
* a readme with actual info