Small fix for parsing tracker URL.

Fixed and improved some small problems in converting tracker URL. All tracker URLs will be converted into a uniformed format listed as follows:

(1) HTTP tracker URL should be:  http://\<host\>:\<port\>/\<announce\>
(2) UDP tracker URL should be:  http://\<host\>:\<port\>

This operation not only validates a given tracker, but also removes unnecessary duplicates.