XD-647 Refactor message converters to implement MessageConverter interface

XD-647 merged ModuleTypeConversionPlugin

XD-647 Http source emits byte[] by default

XD-647 General cleanup, added custom converters hook

XD-647 Revert to http source emitting String by default, and Javadoc

Fix generics warnings

Bump groovy version
@Ignore a failing test for now pending investigation