Weighting Schemes: BM25+ implementation

Hi Olly,

You might want to take a final look at the BM25+ implementation in this
pull request here (https://github.com/xapian/xapian/pull/104). It's
functional with all tests passing. Had fixed whitespaces issues soon after
discussion on IRC couple of weeks ago. Please do let me know if any
additional changes are required before it can be merged.

Also, PL2+ implementation is complete and I had opened a PR few days ago
(awaiting review). Please have a look at it too :)

Thanks,
Vivek