A tool to run local perceptual diffs

This is a proof of concept for a local pdiff test using dpxdt.

Here's the usage:

    $ ./dpxdt/tools/local_pdiff.py test dpxdt/tools/local_pdiff_demo
    homepage: Running webpage capture process
    homepage: Resizing reference image
    homepage: Running perceptual diff process
    homepage passed (no diff)
    different-text: Running webpage capture process
    different-text: Resizing reference image
    different-text: Running perceptual diff process
    different-text passed (no diff)

If you add a box shadow or something to `dpxdt/tools/local_pdiff_demo/index.html` and rerun:

    $ ./dpxdt/tools/local_pdiff.py test dpxdt/tools/local_pdiff_demo
    homepage: Running webpage capture process
    homepage: Resizing reference image
    homepage: Running perceptual diff process
    homepage failed
      0.0103501 distortion
      See /var/folders/9q/340_463n2fvbzjg232s3hrt00000gn/T/tmpNVxkQK/tmpUXP6UC/diff.png
    different-text: Running webpage capture process
    different-text: Resizing reference image
    different-text: Running perceptual diff process
    different-text failed
      0.0103501 distortion
      See /var/folders/9q/340_463n2fvbzjg232s3hrt00000gn/T/tmpNVxkQK/tmpiolKAG/diff.png

... and then you can update the reference images:

    $ ./dpxdt/tools/local_pdiff.py update dpxdt/tools/local_pdiff_demo
    homepage: Running webpage capture process
    homepage: Updated dpxdt/tools/local_pdiff_demo/homepage.png
    different-text: Running webpage capture process
    different-text: Updated dpxdt/tools/local_pdiff_demo/different-text.png

Thoughts much appreciated! If this is on the right track, I can add tests and such. There's definitely some duplicated code that could be consolidated, too.