tests: run BrowserStack in parallel with Sauce Labs

Per https://github.com/twbs/bootstrap/issues/11087#issuecomment-31922999
and internal twbs team discussion.
Effectively reverts the deletions from #11701.

Gruntfile.js

@@ -351,6 +351,7 @@ module.exports = function (grunt) {
 
   // These plugins provide necessary tasks.
   require('load-grunt-tasks')(grunt, {scope: 'devDependencies'});
+  grunt.loadNpmTasks('browserstack-runner');
 
   // Docs HTML validation task
   grunt.registerTask('validate-html', ['jekyll', 'validation']);
@@ -372,6 +373,12 @@ module.exports = function (grunt) {
     testSubtasks.push('connect');
     testSubtasks.push('saucelabs-qunit');
   }
+  // Only run BrowserStack tests if there's a BrowserStack access key
+  if (typeof process.env.BROWSERSTACK_KEY !== 'undefined'
+      // Skip BrowserStack if running a different subset of the test suite
+      && (!process.env.TWBS_TEST || process.env.TWBS_TEST === 'browserstack-js-unit')) {
+    testSubtasks.push('browserstack_runner');
+  }
   grunt.registerTask('test', testSubtasks);
 
   // JS distribution task.