Decouple Rules and Callback handlers

Right now an IRule does two things:

• Describes the matching rule to the parser
• Provides the parse event handler

Originally this design seemed beneficial because it lead to a smaller API, but as I learned from working on the CloudFront Log Parser, decoupling illogical pieces can be a beautiful thing and lead to some nice enhancements.

Work for SJXP 3.0:

Decouple IRule into

• ICallback (implements the handler logic)
• IRule (only specifies the matching conditions AND the callback to be invoked)

Other enhancements due to these changes will be filed separately.