Image comparison using ImageMagick's "compare"

An attempt to address issue https://github.com/Khan/KaTeX/issues/143.  I've moved all of the reference images from the Huxleyfolder into a sibling folder called "originals".  When huxley runs it create new images in the Huxleyfolder.  To compare the images, run ```node compare.js```.  The script calls ImageMagick's ```compare``` command so ImageMagick will need to be installed for this work.  Fortunately, ImageMagick is part of the travis-ci environment so it should be easy to automate this.

Right now it spits out text the console in the following format:
```
BasicTest: 0 @ 0,0
FractionTest: 0 @ 0,0
NestedFractions: 0 @ 0,0
...
```
The script use the ```-subimage-search``` to deal with differences in image size arising from a bug in node-huxley: https://github.com/chenglou/node-huxley/issues/76.  It also uses AE (Absolute Error) to calculate the error.  There are other metrics available: http://www.imagemagick.org/script/command-line-options.php#metric.
At one point I was trying to use plain-old selenium and it almost worked expect for one test failing for reasons that I couldn't explain (even after getting the right version of freetype).  When selenium takes screenshots, by default the background is transparent.  There's probably some setting that you can use to change it.  It's no longer necessary.