merge download changes from nautilus-dropbox-1.6.1/dropbox.in

http://linux.dropbox.com/packages/nautilus-dropbox-1.6.1.tar.bz2
init self.task to None

merged changes from nautilus-dropbox 1.6.1 to 1.6.2

this bug happens when you try to press cancel on first dialog
Merge pull request #14 from glensc/dl-fixes

update download code from nautilus-dropbox-1.6.2