Merging with master

 - Changing from result method to get_latest_result because it an array
now.
 - Deletion of return statements
 - Trying to respect the 80 chars
Merge remote-tracking branch 'origin/master' into issue-798
Closes 997: Allow multiple files

 - Just adding of the attribute multiple
Deleting of my warning
Closes #474: Move custom validations to lib

 - Application of the patch
Merge pull request #1119 from GhiGt/issue-474

Closes #474: move custom validations to lib

We put our custom validators in a library validation class and then use validates.
Merge pull request #1100 from GhiGt/issue-798

Closes #760 : TA cannot choose 'filter' attribute properly
Merge pull request #1113 from GhiGt/issue-997

Closes 997: Allow student to send multiple files.
Closes #191: Remove index in annotation_categories

- Remove of last migration
addition of a message
Merge pull request #1121 from GhiGt/issue-191

Closes #191: Remove index in annotation_categories
Merge branch 'master' of https://github.com/GhiGt/Markus