Allow configuration of iframe src attribute
Create new event objects for each fileupload event trigger.

The original jQuery event can be accessed as delegatedEvent property.
Support preventDefault on the event callbacks to disable default behavior. Closes #2651
Merge pull request #2652 from bmwyant/master

Allow configuration of iframe src attribute
Merge branch 'master' of github.com:blueimp/jQuery-File-Upload
Added new initialIframeSrc option, based on pull request #2652.