SWIG bindings and documentation for sbml2matlab.

Also, no longer ever just 'exit' from routines, since that's an evil thing to do from a library.

New API function 'getMatlab', which takes SBML as an argument and returns a matlab string.  This is a much easier function to wrap than the 'sbml2matlab' function.  In the bindings, renamed 'getMatlab' to 'sbml2matlab', assuming that would be the expected function to find.

Also, made some validation functions take const arguments.