get_url module: added use_timestamp with similiar behavior like ants get task

the behavior of get_url is pretty stupid. What I would expect is something like the get task from ant. I mostly use directories as destinations in my playbooks, and I have limited bandwidth, so always downloading the files again and again is annoying.

I added use_timestamp an a way that the behaviour without it doesn't change, but when used it will not download the file again even if it is in a directory.