We should have an automated performance test suite

[richard]

We need to be able to keep track of how changes to the code affect the performance (ie, speed / resource usage) of Xapian. In particular, we should be able to test how fast a standard set of data is indexed and searched, simply by running a single command (ideally, integrated into the build system - eg, "make speedcheck").

I have the beginnings of such a system, in the shape of some python code which builds a wikipedia index. I'm starting this bug to keep track of progress on building this.