Implement JSON file based asset pipeline

Adds manifest.json which contains all of the mappings for the assets
ref: https://github.com/roots/roots/pull/1138#issuecomment-62593715

Prune and update NPM deps

README.md

@@ -74,9 +74,8 @@ When completed, you'll be able to run the various Gulp commands provided from th
 
 ### Available Gulp commands
 
-* `gulp` — Compile LESS to CSS, concatenate and validate JS
+* `gulp` — Compile and optimize the files in your assets directory
 * `gulp watch` — Compile assets when file changes are made
-* `gulp images` — Lossless compression of PNG, JPEG, GIF and SVG images
 * `gulp --tasks` - Lists all the available tasks and what they do
 
 ## Documentation