SauceLabs browsers badge

- add the Sauce Labs browser badge (`reporters: ['progress', 'saucelabs'],` in Gruntfile and badge in Readme)
- make the test name on Sauce more verbose `Gruntfile.js - L86`
- refactor SL credentials so we use sub-accounts, which will make possible other repos to be tested and have own badges
- update NPM version for `karma-sauce-launcher` because they had a bug in previous versions for the reporter which reports back to SL if the test passed or failed

The idea is to make it look [like this](https://github.com/SergioCrisostomo/mootools-core)