Various enhancements to gulpfile

* Run `bower install` each time gulp runs
* Add `gulp-plumber` to keep `gulp watch` from dying
* Fork `gulp-manifest` plugin and improve it
Also run bower before polyfills
Run tests with `npm test`
Use commit number for gulp-manifest instead of branch
Merge pull request #269 from scrollback/smallfixes

Various enhancements to gulpfile