Add once, throttle and throttle_identical log variants

[romainreignier]

Hi, I have tried to implement some log variants (throttle, throttle_identical and once) by following the advice in #30 I am not a Rust power user so feel free to comment or re-implement yourself if you wish.

• There are some repetitions between throttle and throttle_indentical but I have prefered to not copy the message when not needed.
• For the throttle_indentical, rospy stores a hash of the message. Do you think we should do the same? It might help for large strings I guess.
• I am not sure if my usage of lazy_static is correct