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.
I originally had "sleep 2".  I copied it from the huxley dockerfile.  Then I was talking with one of the devs from travis-ci team about my problems and he suggested sleeping for longer after the server starts up (initially he thought that the server was running and huxley had troubles connecting).  I'll switch it back to 2 seconds.