Pipeline phases documentation

I've attempted to extend the documentation of pipelining phases by creating an example of an integer reverser. I've not included stuff such as progress reporting, since I wanted to keep the example as small as possible.

Also, I've made the `tpie::stack<int>` pointer a parameter of the constructors of `reverser_input_type` and `reverser_output_type` instead of forwarding it in the propagate method of `reverser_type`. Is there any reason to pick one method over the other?