Introduce webmock to stub HTTP request hitting real Flickr API for specs.

There will be two set specs:
1. Mock HTTP specs: Flickr API data will be served by local json files; this will speed up the tests, and gives reliable tests result during development.
2. Live HTTP specs: Flickr API request will hit real end point and fetch real data; these specs should be run before release to production to make sure live search is working.