[plt-scheme] PLaneT now supports HTTP downloading

The version of the PLaneT collection in Subversion now supports 
retrieving PLaneT packages using the HTTP protocol. It can still 
download packages using the old protocol, but now by default it uses 
HTTP. The parameter USE-HTTP-DOWNLOADS? described in the planet 
collections's doc.txt file controls what protocol to use, but unless one 
protocol or the other doesn't work for you, there's no reason to care.

This change was made because of numerous problems people had with 
firewalls that only allow HTTP traffic; hopefully these people will now 
be able to use PLaneT more easily.

This is experimental, and I'd love to hear feedback on whether it works 
for you.

-jacob