Change unit testing to use node.js instead of Rhino.

A few notes:

- Running tests in node.js is *way* faster! (For me: 0.5 seconds instead of... 43 seconds in Rhino.)
- The linter still uses rhino. I would add `make lint` to the tests, but that currently fails.
- Travis [runs `npm test` by default](http://about.travis-ci.org/docs/user/languages/javascript-with-nodejs/), which requires adding a `test` command to the NPM package file. That seemed reasonable.
- I've checked that `browserTest/browserTest.html` still works.
- I've checked that the test output is the same except for timing info, and [correctly reports failures](https://travis-ci.org/lgarron/sjcl/builds/12482286).
- node.js 0.6 is [being unreliable](https://travis-ci.org/lgarron/sjcl/jobs/12481982) on Travis at the moment, so I took it out. This means testing against node 0.8, 0.10, and 0.11, which should be fine.
- Test failures are still not very informative. They'll have to be debugged locally either way, but some stack traces might be nice.