Automated cross-browser testing

Initial take on: https://github.com/tildeio/htmlbars/issues/86

This PR implements cross-browser testing for all commits to master (no PRs yet) using SauceLabs. Included browsers are IE 8 - 11 and the two most recent major versions of Chrome, Firefox and Safari.

Test time is currently around 10min, but this should be improvable by running multiple browsers in parallel (tried, but the testem -P option seems to be broken)

Example build: https://travis-ci.org/johanneswuerbach/htmlbars/builds/44101702

Currently tests in IE 10 and 11 are failing:
https://saucelabs.com/tests/1c70452ed95f4a3f981ea91f8bf65c9a
https://saucelabs.com/tests/e4f79e5df8b542ef84afe58706a9eef1

The SauceLabs UI shows only details of failed tests, as otherwise the a 65KB limit on Sauce is reached and tests are marked as failed by Sauce ....