Implement ValueWeightPostingSource class bindings

endlessm/eos-sdk#2009
this should be replaced by an internal accessors pair:

    Xapian::PostingSource *xapian_posting_source_get_internal (XapianPostingSource *source);
    void xapian_posting_source_set_internal (XapianPostingSource *source, Xapian::PostingSource *internal);

which concrete subclasses are supposed to call during initialization.

see also the `XapianStopper` class that @wgreenberg implemented recently.