Unit testing for User, Experiment, and Experiment_sessions: Validation of default fields and creation of fixtures

Created a new User, Experiment, and ExperimentSession and tested that all the default fields were set to their correct defaulted values. Verified that a new user has email defaulted to nil, validated to false, admin to false, and content to nil; a new experiment has validated defaulted to false, admin to false, and content to nil; and a new experiment session has content defaulted to nil.  Unit files modified include experiment_test.rb, experiment_sessions_test.rb, and user_test.rb.

Additionally, I created a fixture for experiment, experiment sessions, and user and tested that the fields in the fixtures were correctly set using assert_equal or assert_nil.  Fixture files modified include experiments.yml, experiment_sessions.yml, and users.yml.