Closes #1073: Possibility to import/export assignments

 - add upload and download link
 - add actions in AssignmentController
 - modification of required fields for an assignment

/!\ Some modification are still needed
added encoding with iconv
added some tests and fixed a bug
Merge branch 'master' into issue-1073
somes small changes
Merge branch 'master' into issue-1073
Merge master
last changes

- fix bad i18n
fix bug
Merge pull request #1127 from GhiGt/issue-1073

Closes #1073: Possibility to import/export assignments