When there is no marks self.total_mark is 0. Condition is used to avoid division by 0.
Fix issue #1388
New test: if an assigment has no marks and was completed and released to student should has values median and average results values equal to 0. 

Equation that divides by total marks (0 if no marks in the assingment) should not be reached.
Correcting lines that are setup and verification.
Correcting comment.
Verify if students_counts is not zero, avoiding division by zero
Fix: schema.rb
Merge branch 'issue-1333'
Merge branch 'master' of github.com:MarkUsProject/Markus
Merge branch 'master' of github.com:MarkUsProject/Markus
Merge branch 'master' of github.com:MarkUsProject/Markus
Merge branch 'master' of github.com:MarkUsProject/Markus
Calendar_date_select: Using a gem instead of vendor/plugins
Removing vendor/plugin calendar_select date and installing it from gem.
Command "bundle exec rake calendar:install" should be execute for correct installation.
Adding gemfile.lock after setting calendar as a gem.
Rebasing branch into master
Merge pull request #1394 from tiagochst/issue-1388

Closes #1388: when marks are not used in assignments, releasing marks produces an error.
Merge branch 'master' of github.com:MarkUsProject/Markus
Remove SSL requirement
Merge branch 'issue-1324-calendar'