Add some helper classes to allow easier polymorphism between string types.

Implicit conversions allow specifying strings as "const char *" or
"std::string".  Additionally a numeric overload allows specifying the
port as an "int", converted automatically to a string for liblo.
Use std::initializer_list rather than std::vector for arguments.
Remove defaults for proto argument to disambiguate constructors.
Simplify handler method type declarations somewhat.
Remove return value from bundle handlers, since they don't do anything.