include bower install as npm postinstall script

* See scripts section https://www.npmjs.org/doc/json.html
* They prefer for your setup to be done when npm install is done.
  See several issues for weirdness surrounding deployment scripts.
  See https://github.com/projectkudu/kudu/wiki/Custom-Deployment-Script
  and other material for their preferences.

This should make it so that npm install also runs bower install.

Tested by deleting everything, checking out, and running `npm install`.
This time, bower install ran automatically, as part of npm install.