Omega: Missing support for newer weighting schemes

[Vivek Pal]

> enq.set_weighting_scheme(Xapian::BM25Weight());

Sorry; it should be Xapian::PL2Weight() here.

> enq.set_weighting_scheme(*wt->set_parameter_values(p));

set_parameter_values is basically parse_params method; an attempt to
find a better name for parse_params.