Download count feature

*Title: Download count feature

Incentive:
We need to know how many times a file is downloaded.

*Documentation:
New field download_count in the ezbinaryfile mysql table.
It's the number of downloads of the related file.
Changes done in ezbinaryfile.php and ezbinaryfilehandler.php
To use it in a template , use file.content.download_count

*Testing:
1- Add a file in a folder
2- Download it several times from different computers
3- Display the number from a template (file.content.download_count) and check the number increases
(you can do it directly from mysql with a request)
4-Try the same with a non existing file (or delete the previous one) to check that the download numbers do not increase in this case

*SVN revisions: