Fix memory leaks in toString methods

 - Memory allocated into a gss_buffer_desc should be released via
   gss_buffer_release (yes, it's just doing a free(3) under the hood
   but it might not do that forever).

 - Methods that return char* to be marshalled into Java Strings need
   to be marked %newobject so that the memory will be cleaned up after
   constructing the String.