Externalizes build process with pixify module (#2778)

Converts to NPM scripts, removes Gulp (#2769) ...

* Convert to NPM scripts, removes Gulp

* Requires node bump for jshint-stylish-summary

* Missing mkdirp

* Fixes bundling sourcemap problem

* Fixes linting

* Bumps Travis to supported NodeJS version

* Typo

* Fixes build script for Windows

* Fixes watch on Windows

* Removes unused dependency

* Remove unused dep