Decorators

This contains the definition of the decorators that may be used on a
processor function.  The @want() decorator may be used to select which
events the processor function should be called for; the @requires()
decorator may be used to specify other processors that should run first;
and the @required_by() decorator may be used to specify that the
decorated processor should run before the listed processors.
Simplify use

This addition to __init__.py pulls all the important, non-internal
symbols into the package namespace, simplifying the use of EvProc.