Add Behavior class as in C# version

[the-real-blackh]

We're doing this on all languages:
https://github.com/SodiumFRP/sodium/blob/master/c%23/src/Sodium/Sodium/Behavior.cs

[clinuxrulz]

Just to clarify.
Cell: The discrete one.
Behavior: The continuous one?

[the-real-blackh]

Yes, that's right.

[the-real-blackh]

jam40jeff added this comment to the Java ticket:
The Behavior class is mostly a rename of the Cell class with a couple methods removed. The new Cell class contains the discrete cell behavior (and uses an underlying Behavior for its implementation in C#).