SlimerJS support

Work in progress. This addresses #286 and I would like to discuss how to continue.

The run-testsuite.js has been altered to use the http:// protocol of a static file server (Python SimpleHTTPServer, Node.js http.server). The .travis.yml has been altered to run the test suite via engine PhantomJS and SlimerJS. This leads to two problems:

- Some tests fail with SlimerJS, I haven't checked yet if this is a problem in the code base or in SlimerJS support.
- SlimerJS does not return the correct exit code on error. This is a [known bug](https://github.com/laurentj/slimerjs/issues/50).