IE10+ compatibility

Prevent default on dragenter so it can work with IE10+
Tested on IE10 (doesn't work with IE9 and lower)
Tested on Chrome
Merge pull request #3331 from darknessm0404/patch-1

dropZone fix for IE10+ compatibility
Refactored last merge. Closes #3321
IE10+ drag&drop patch release.
Merge branch 'master' of https://github.com/blueimp/jQuery-File-Upload