zanata_stats: refactor
zanata_stats: refactor

* Convert more functions to class methods.
  It allows us to have related stuffs in one place.
* Avoid using 'yield'. 'yield' itself is useful,
  but there is no need to use this for small iterations.
  Code readability will be better if we return a list.
* Raise an exception when an exception occurs in
  read_uri() and read_json_from_uri(). Catching an exception
  in these methods just leads to other unexpected error.
  If reraising an exception brings, debugging will be easier.
* Move the sort logic to User.__lt__ method.
* Load language team YAML file after checking options.

Change-Id: I9b668e4606f783fbb87e920a4802947c7efe4eb3

openstack/i18n,openstack/i18n