Improve testing

Summary:
Move dom creation into katex.js so our tests can test non-dom things, and add
some buildTree tests. Add some checks make utils.js work in node. Add support
for jasmine-node, to allow for command line unit testing.

Test Plan:
- Make sure tests work, in both the browser and with `make test`
- Make sure huxley screenshots didn't change

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D13125