web-cmd.py: add --browser option to open the site

Signed-off-by: Holger Dell <github@holgerdell.com>
[rlb@defaultvalue.org: adjust commit message]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
web-cmd.py: open browser from ioloop callback

Assuming the callback won't be called until tornado is ready to
receive requests.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>