added model files for each model
created user model validations and associations
created question model validations and associations
created tag model validations and associations
created vote model validations and associations
created answer model validations and associations
created comment model validations and associations
created questions_tags model validations and associations
configured postgres in database config
added pg gem
added spec helper, capybara, and database_cleaner for testing
created tests for user model associations and validations
added schema
added answer models tests
added question models tests
added vote model tests
added tag model tests
added comment model tests
added questions tag model tests
added session class