Added Apoorv Upreti's GSoC2013 work: visual test suite.

See visualtest/README.txt for details.

visualtest/testsprite2_sample.actions

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+00:00:02 SCREENSHOT # Take a screenshot of each window owned by the SUT process
+00:00:05 VERIFY # Verify each screenshot taken with verification images
+00:00:10 QUIT # Gracefully quit the SUT process