Tidy up matchspy.h for inclusion in a release

[richard]

matchspy.h currently contains a set of matchspy implementations, and some associated functions, which are not yet in any release. We need to go through and tidy these up before the next release, before committing to supporting these.

Current status:

StringListSerialiser? is a bit ad-hoc - maybe it would be better simply to make values capable of storing multiple values. (See bug #198).

TermCountMatchSpy? should probably be removed - it goes against the explicitly documented "Match spies and match deciders shouldn't access anything except for document values", and is thus liable to be misused, leading to a bad impression of Xapian's speed. Richard is doing some tests to see how performance compares with a value based implementation (but this would also be helped if it was easier to store multiple values in a document).

CategorySelectMatchSpy? possibly shouldn't be a match spy - it could simply be a class (or a function) which is passed the results of ValueCountMatchSpy::get_values(), and returns a score for the categorisation, together with another function which converts the output of get_values() to a set of ranges.

[richard]

Olly's backed the matchspy stuff out of SVN HEAD for now, so we've got until the 1.0.4 release to tidy this up. Changing to block #200 instead of #154.

[olly]

Moving to 1.1

[olly]

Reword summary