Add a browsersync example

[nicholaswmin]

Add an example of how to properly hook up browser-sync to work with the custom build.

There's an S.O question about this already but the person that answered it loaded his gulpfile with no less than 20 dependencies which makes it bloated and unnecessarily complex to follow.

[nicholaswmin]

Workaround here: https://github.com/Polymer/polymer-cli/issues/230#issuecomment-233871759