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.