Add parameter category qualifier "forward" (current qualifiers are "in", "out", and "in_out") based on http://www.modernescpp.com/index.php/c-core-guidelines-how-to-pass-function-parameters. Add new usage syntax BOOST_PARAMETER_NAME((object-name), namespace-name) qualifier(tag-name)) and BOOST_PARAMETER_NAME(qualifier(name)). (Existing code that uses qualifiers directly and correctly with BOOST_PARAMETER_FUNCTION and other code generation macros should remain unaffected for now, so no breaking changes.) The reason for the change in usage is to enable applying of parameter category constraints to Boost.Parameter-enabled functions and constructors invoked through argument composition. (Otherwise, it is currently possible to use argument composition to bypass parameter category constraints applied in BOOST_PARAMETER_FUNCTION et. al.) See "test/compose.cpp" for example usage.
