Signal treatment

I have made some treatment to signal. 

* `mutable.Signal` no longer inherits from `immutable.Signal`, instead from `generic.Signal`
* immutable signal can be obtained from mutable signal by `view` combinator
* signal _stamps_ are now wrapped in `discrete` and `continuos` processes, `changes` is now expressed as combination of `discrete` and `continuos` processes. 
* immutable signal has now `map`, `flatMap`, `pipe`, `tee`, `wye` and most of the process1 combinators that (I believe) have some sense on Signal

@pchiusano  can you take a quick look on that before merge?