update class diagram docs
Merge pull request #125 from camallen/update_class_diagram

update class diagram docs
do not allow nil subject_ids to be tracked
set up default emtpy array for user seend subjects subject ids
track which subjects a user has seen and submitted classifications for.

Tried to be as efficient as possible in the request / response cycle for classification create. We can look to background the UserSeenSubject add seen subject method if this is taking too long.
explicitly test the subject_id was stored in the DB
gracefully handle invalid subject id's when updating user_seen_subjects
use ruby 2.0 keyword compatability
tell don't ask
better example description
Fix default table values in JRuby