Download copies

allow to download drops of an evaluation

- only finished drops are downloadable
- only documents dropped are downloadable ( no comments, no corrections yet )

you can give a period to download only drops that have been completed in this period.
![menu](https://cloud.githubusercontent.com/assets/3479942/4541896/c96da372-4e1a-11e4-8f8a-09208e595a68.png)
![period](https://cloud.githubusercontent.com/assets/3479942/4541897/c982eb10-4e1a-11e4-9e6f-6c1b868cca30.png)

the download is an archive that have the following structure :
--> name of the workspace
----> copy N°x - Username + date of the drop
--------> document1
--------> document2
----> copy N°x+1 - Username2 + date of the drop

close #104