Make our own screenshotting script instead of using huxley

Summary:
Create our own screenshotting script which takes screenshots. This
improves over huxley for a couple reasons:
 - It makes the screenshots the correct size (for some reason, huxley struggles
   with this).
 - Its configuration matches more with what we want (we don't need multiple
   screenshots or interaction, we just want a single static shot)
 - It runs faster

I also changed the docs to reflect this change.

Test Plan:
 - Make sure all of the tests that were in the Huxleyfile are now in ss_data.json
 - Run the screenshotter docker
 - Make sure all of the images look reasonable and don't change (except
   sometimes the Lap test, which has some strange pixel-positioning
   differences...)

Reviewers: kevinb

Reviewed By: kevinb

Differential Revision: https://phabricator.khanacademy.org/D16731

package.json

@@ -17,7 +17,6 @@
     "browserify": "~2.29.1",
     "clean-css": "~2.2.15",
     "express": "~3.3.3",
-    "huxley": "~0.8.1",
     "jasmine-node": "git://github.com/mhevery/jasmine-node.git#Jasmine2.0",
     "jshint": "^2.5.6",
     "less": "~1.7.5",