As zuul consequently fails when running on node v0.12.

Use latest v2 release of npm as the just released v3 doesnt seem to be node v0.8 compliant atm.

And test on node v4 for obvious reasons.

As secure $ENVs are disabled for pull requests and those $ENVs are required for running browser tests on saucelabs.

http://docs.travis-ci.com/user/pull-requests/#Security-Restrictions-when-testing-Pull-Requests

+ updated readme with the fact that browser tests are only runned automatically when *merged into master*, not in the PR stage

With 25 different browser versions it could easily take 15-25 minutes for a run too finish.
Only test latest chrome & firefox atm, increase support later as needed.

Removing hacks to get v0.6 tests running; specific npm client version + upgrading mocha.
Also v0.12 of node has been released, therefore there's no reason to still keep testing on v0.11.

Covers the project with browser tests, not only V8 via node/iojs. Tests are runned with [zuul](https://github.com/defunctzombie/zuul) which
provides simple local browser testing, aswell as cloud testing on multiple browsers with [saucelabs](http://saucelabs.com).

- Documented required version of npm while developing
- Use the latest npm client for node >= v0.8 on travis

Special quirks to get node v0.6 supported:
	- npm client v1.3.26 on travis
	- jshint 2.4.4 as >= 2.5.x breaks

package name is now "xulrunner-2.0". We need this to run our tests
in SpiderMonkey.