Closes #570 : Assignment due date with sections

- Add error message to show which section(s) is in past
- Add method to know the sections' names
- Modification of past_due_date? which doesn't work in some cases
Add tests for the new assignment method
Correction of the error in unit tests

 - Change comparison values
Correction of find deprecated
Merge pull request #1097 from GhiGt/find-deprecated

Fix: find(:first) and find(:all) are deprecated 

find(:first) and find(:all) are deprecated in favour
of first and all methods. Support will be removed
from Rails 3.2.
Merge pull request #1096 from GhiGt/error-units-tests

Fix: error in unit tests

That is somewhere, when the object TimeWithZone is created, all content is put except usec.
I didn't find where is the mistake. I look for on the web and some people who have the same problem say that we can just test equity with object.to_a.
Closes #860 : Sort assignments in dashboard

 - Adding a order of assignments in the MainController
 - Adding a test
Merge pull request #1082 from GhiGt/Closes-#570

Closes #570 : Assignment due date with sections
Merge branch 'PR-862'