add google analytics tracking (prod and develop)
On branch develop
Your branch is up-to-date with 'origin/develop'.

Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

	modified:   _config.yml
	modified:   _data/css_and_js.yml
	new file:   _development_config.yml
	modified:   _layouts/default.html
	new file:   javascripts/ga.js