Add a small TODO
Use a channel for limited parallel downloads

Actually this code would be obsolet if WaitGroups would export more data
like their counter for example.
Remove debug output
Don't create an empty file if the download fails

But do create the target directory.
Yet another TODO
Use a for loop to determine if another download can be started.

Just in case the counter is off by more than one, which shouldn't
actually happen.
Don't use parentheses