Assessments models,  migrations and seeds for Quiz

Created models Category, Quiz, Question, Answer and Tag.

If you need to make any changes to DB, create new migration file 
`rake db:create_migration NAME=your_name_for_file` and use `rake db:migrate` to apply changes.
For seeding use `rake db:seed`
More info on https://github.com/janko-m/sinatra-activerecord