Rename new_user method to create_user to match CRUD actions.

Replaced new_user method name with create_user.
Added stub method with deprecated notice for new_user.
Update method name to create_user in user tests
Fix stub method name of what should be new_user
Revert test variable names in post and user tests.
Merge pull request #376 from WP-API/issue-376

Rename WP_JSON_Users::new_user to create_user