Rewrote `Message` object to use a recursive structure `MessagePart`.
Removed `Message`, `Request` and `Response` as they're now unused.
Updated `FunctionProvider` and `MessageFormatter` superclass code
templates to use `MessagePart`. Updated FP and MF code templates to match.
Rewrote `XMLFormatter` implementation to build/parse
`MessagePart`s.
Updated `Endpoint` code template to handle `MessagePart`s

refs #31, #28