RIP invoke_rules

The class invoke_rules was among the first classes of libcppa. In fact, it received a lot of refactoring even before the first commit on github. However, it's finally gone. If your code fails to compile with the current version, this is how to fix your code:

invoke_rules → partial_function
timed_invoke_rules → behavior

The class invoke_rules had too much changes in the past and its name isn't very well chosen. In fact, the implemented behavior of it already was identical to a partial function. But it had a brother called timed_invoke_rules that was a partial function with a timeout. That's pretty much the definition of an actor's behavior, isn't it? It's an old remains from the time I've implemented on() and after(). The new partial_function/behavior interface is straightforward and much more intuitive.