Add Bumbleworks.entity_classes

This array is accessible via configuration, but will be automatically
appended to when you include Bumbleworks::Entity in a class (the
including class will be appended).
Add Expression and Process::ErrorRecord wrappers

An Expression instance wraps a FlowExpression, for a more logical API.
A Process::ErrorRecord instance does the same for Ruote::ProcessError
instances, also cleaning up the error message and adding the ability
to #reify the error (re-instantiate the original error).  The Process
class has been changed accordingly, to return the new classes for
relevant queries.
Add more helper methods (esp. for reports/GUI)
Add test for new Expression#kill! method

We should add tests that actually check that the cancel!/kill! methods
succeed, with real expressions.