[patch][merge] sftp/http tweaks

[Robey Pointer]

Since my bzr.robey branch only had 2 un-integrated patches in it  
(when compared to bzr.dev), I blew it away, did a fresh branch from  
bzr.dev, and applied just those 2 patches.

1516: change http url parsing to use urlparse, and use the ui_factory  
to ask for a password if necessary
1517: allow forcing the use of paramiko via environ var; use prefetch  
on paramiko >= 1.5.2"

So I'd like to resubmit those for merge and/or feedback.

The branch is at http://www.lag.net/~robey/bzr.robey/

robey