Added support for on_pre_[action] methods to Ruote::ProcessObserver

Hello,

I enhanced Ruote::ProcessObserver to support the ability for a subclass to define on_pre_[action] methods.  My application has the need to do something just prior to a specific message being processed.  I originally implemented an on_pre_msg method in my application's Ruote::ProcessObserver subclass, however I found that I had to duplicate all the code from Ruote::ProcessObserver#on_msg that sets up the wfid and info parameters.  I felt that a better solution was for Ruote::ProcessObserver to implement an on_pre_msg, by having it share code with the existing on_msg method.  

I also added a couple of tests for my changes to test/functional/ft_77_process_observer.rb, and then ran the full suite of tests to make sure they still pass (which they did).

Please consider accepting my pull request.

Thanks,
Denis Ahearn