Added database models for the site_mean_and_targets and
site_moving_averages table.

Created a new view which will retrieve data from these two tables by
constituent and site_id and return a json object. Updated the tests to
test this new view. Added a url conf which will use this new view.
Merge pull request #255 from mbucknell/NAWQAAN-65

Added database models for the site_mean_and_targets and