Declare factory methods with %newobject

Add the %newobject SWIG declaration to the factory methods createLattice()
and createTagger() of the class MeCab::Model. This makes it clear that the
caller takes on ownership of the pointer returned by these factory methods.

Without this declaration, calls in e.g. Java to Model.createLattice() and
Model.createTagger() will leak memory, as the generatedd Java Lattice and
Tagger proxy classes do not know that they have ownership of the underlying
C++ pointers. As a result, instances of the proxy classes fail to delete
the underlying C++ objects, leading to native (off-JVM-heap) memory leaks
that are difficult to debug.

mecab/swig/MeCab.i

@@ -14,6 +14,8 @@
 %}
 
 %newobject surface;
+%newobject MeCab::Model::createLattice;
+%newobject MeCab::Model::createTagger;
 
 %exception {
   try { $action }