change random-users

- provide POST version of random-users in API
  - change caffeine_api_v1.tests.test_views.RandomUsersTest
  - change caffeine_api_v1.views.random_users
- provide GET version of random-users in caffeine.views
  - move json_response decorator to new module core.utils
  - implement random_users in caffeine.views
  - add caffeine.tests.test_views.RandomUsersTest
  - add core.tests.JsonResponseTest
- use GET version of random_users from caffeine.views in explore.html
document REST API v1 using sphinxcontrib.httpdomain

- add sphinxcontrib.httpdomain to requirements/local.txt
- add documentation for API based on existing documentation for original
  coffeestats
- add sphinxcontrib.httpdomain to extensions setting in conf.py