The @timer decorator can be called with optional arguments

In need of passing some init arguments to the Timer class, you
can pass these parameters to the dectorator.
Ex: ``@timer(factor=1000)``

Otherwise, you can just use the decorator with no parentheses at all,
simply passing the decorated function to it.
Ex: ``@timer``
Signed-off-by: Balthazar Rouberol <balthazar.rouberol@mapado.com>
README update

Signed-off-by: Balthazar Rouberol <balthazar.rouberol@mapado.com>