Stopper

Implements Xapian::Stopper and Xapian::SimpleStopper, fixes #11 
you shouldn't be using `g_initable_new()` here: neither `XapianSimpleStopper` nor `XapianStopper` is a `GInitable` implementation. just call `g_object_new()`.

this also means that you don't need the `GError` argument to the constructor.