Update Location and Vendor stores to point at staging API endpoints.

Prior to this commit, all data in the app had been fetched from sample
.json files. This commit points the Locations.js and Vendors.js stores
to the correct API endpoints and updates the corresponding store tests
in Jasmine.

This update implies further work:
- Fix stores to include a "empty" selection such as "please choose a product"
- implement autoload callbacks.
- Update the Location model to match the API output