Added PHP documentation, built using ApiGen.

This involved some tweaks to the code comment blocks, using a
combination of Javadoc tags and markdown/reST markup.

Also needed to fix the generated results in a few places (did also
try phpDocumentor, but the results were nowhere near as good and
would have required a lot more fixing up).

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Imported using git-svn from
https://dev.csi.cam.ac.uk/svn/lookup/trunk@78