Match performance

An implementation of [pyramid template matching](http://opencv-code.com/tutorials/fast-template-matching-with-image-pyramid/).
This is a performance optimisation for `stbt.wait_for_match` and
`stbt.detect_match`.

The main change is f150f50a; see that commit message for details. This
passes all the stb-tester unit tests, as well as a private collection
of template images + screenshots taken from the YouView UI.

This pull request also includes a change (d8e4718d) to the `stbt run -vv`
mode that dumps to disk intermediate images from the image
processing algorithm. Now it generates an `index.html` file that
presents the images in the correct order, notes the decision taken at
each point of the algorithm, and explains the images somewhat. See an
example at <http://stb-tester.com/stbt-debug-example/detect_match/00001/index.html>.