Remove ancient lib/tornado in favor of an external dependency.

Remove ancient lib/tornado in favor of an external dependency.

Our lib/tornado dates from over three years ago -- drop it in favor of
an external dependency, and update the install instructions
accordingly.

Anyone trying to run "bup web" without tornado installed will see
this:

  error: cannot find the python "tornado" module; please install it

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