Sauce Integration 

Can someone review this, I haven't worked with `karma` before. Tests pass locally, but the sauce component is still untested.

Based the `karma.conf-sauce` file on [angularjs's karma sauce integration](https://github.com/angular/angular.js/blob/master/karma-shared.conf.js)

(BTW Sauce accounts are Jeremy's)
Yap, it [is](https://github.com/lodash/lodash/blob/d7f568dccc354331385eb86bf9f8e427f77e53ff/test/saucelabs.js#L126-L134) [doable](https://github.com/lodash/lodash/blob/d7f568dccc354331385eb86bf9f8e427f77e53ff/test/saucelabs.js#L898-L901), though I'm not sure how to do it via Karma. I think it would make for a good secondary PR.