models for  experiments, experimenters, sessions and join_experiment_experiment_table
inverse_of added to associations : belongs_to, to avoid appearance of dosynchronized copies of objects.
Syntax errors in models and migrations were fixed.
:join_tables mentioned in the manual way, and new schema
added roles (admin, experimenter, subject) for users
Added categories and associations clarified
categories and migrations added
added participated and invited to join table
Actual version of database.
added attribute normalizers for experimenter and categories (slugs == tags)
Remove tests. We'll add them back later
Merge branch 'feature/experiment'
Update Gemfile.lock