added rspec tests
Merge branch 'master' of git://github.com/MarkUsProject/Markus
removed unused Assignment#submission_by(user) from project
corrected example description
fixed houndci complaints
Merge pull request #1862 from kubasub/master

RSpec tests
migrated more rake tests to rspec tests
migrated tests for #valid_groupings and #invalid_groupings
Merge branch 'master' of git://github.com/MarkUsProject/Markus
fixed houndci complaints
Merge pull request #1871 from kubasub/master

RSpec test