[MATLAB] corrected previous change for staticmembervariableHandler

previous update added lines for immutable constants, but they weren't
written as static methods.
[MATLAB] use SWIGEXPORT for mexFunction

this should mean the user doesn't need .def files.
Also tiny update to usage message of the mexFunction to make clear
this is really an internal function and shouldn't be used directly.
[MATLAB] removed SWIGEXPORT again as it generates conflicts on Visual Studio
[MATLAB] Fixed typemaps for std::pair