add help_requests table
add help_requests model, associations
add open field to help_requests
enrollments fixture, add student_3 to term_2
add help_requests fixtures
add tests for validations on help requests
add validations to help requests
Merge pull request #15 from cs198/help-requests

Help requests
use travis postgres for testing rather than heroku
Merge pull request #16 from cs198/travis-postgres

use travis postgres for testing rather than heroku
add validation for nonduplicate user checkins
tests for uniqueness
Return helpercheckin objects instead of people. TODO: nest people
modify routes for helpers
implement helpercheckin create, destroy
implement show
add checkout time to schema for helper checkins
return 200 on deleting already deleted checkin for idempotency; set checked out time
add another active staff member fixture
add create controller unit tests