Add -randomorder to download command.

[Wesley-Chan]

Some of us have bad Internet connection. If the download process is being taken out in alphabetic order every time, the whole download process would be stuck indefinitely at some large games in the middle of the line if we retry the download again and again, while the small games at the end of the line never got a chance to be downloaded. One solution I can think of, is to download them in a randomized order.

This commit adds -randomorder to download command. If so, shuffle is used to shuffle the items list, otherwise it remains untouched as original code.

./gogrepo.py download -randomorder