IDL CPP Backend initial commit with minimal configuration

IDL CPP Backend initial commit with minimal configuration:
- pom.xml
- mind-plugin.xml to override standard idl-backend when --cpp is enabled
- new StringTemplate (strict copy from idl-backend, renamed to cpp subpackage ; file to be modified)
- new IDLCPPHeaderCompiler generator file using the said StringTemplate
- new Guice module descriptor to bind IDLVisitor to IDLCPPHeaderCompiler instead of IDLHeaderCompiler