Add interceptor functions

[doriansmiley]

Allow the central event bus to define interceptor functions. The idea would be that these functions will be called before the event call back functions are executed. Consider the following interfaces signature taken from parsley:

MessageInterceptor: http://www.spicefactory.org/parsley/docs/2.1/api/parsley-spicelib-flex/org/spicefactory/parsley/core/messaging/receiver/MessageInterceptor.html#propertySummary

MessageProcessor: http://www.spicefactory.org/parsley/docs/2.1/api/parsley-spicelib-flex/org/spicefactory/parsley/core/messaging/MessageProcessor.html

The MessageProcessor exposes a proceed method. I think the central event bus should check if interceptors are defined and if so it should create a new instance of a concrete MessageProcessor instance assigning all the callback functions as separate MessageInterceptor instances. The MessageProcessor would then iterate over all the MessageInterceptor instances calling the intercept method of each passing itself. The MessageProcessor's proceed method will then pop the processed MessageInterceptor from the collection and wither call the next MessageInterceptor in the collection or finish execution and call the MessageProcessor's destory method. IMPORTANT: execution of a MessageInterceptor must occur in a try catch block. Also, MessageProcessor's should also expose a cancel method which can also be called by the MessageInterceptor. This will trigger the MessageProcessor's destroy method and clean up.

[ghost]

links provided are broken @doriansmiley :(