Recursion for @boost method

Created originally on Bitbucket by paugier (Pierre Augier)

I don't think there is any big technical issues to support things like

def func(self, arg):
    return self.attr * arg + self.func(arg - 1)

We just need to replace self.func(arg - 1) by something like new_func(self_attr, arg-1)...

In particular, I guess there is no problem with inheritance...