drop builtin html conversion support

Use external XSLT stylesheet for this purpose.
drop force configuration directive
drop local target concept
all targets are servers now
rename target concept to controller
replace XML classes by Message classes

XML::Query::Inventory and XML::Query::Prolog are now replaced
Message::Outbound, and XML::Response is now replaced by Message::Inbound
no more mandatory target parameter in a task
reintroduce target concept, as the results destination
send results to provided target