Automatically install dev dependencies

Makefile

 watch: build
 	fswatch -0 three.js/src/*/*.js aframe/src/*.js babylon.js/src/*.js | xargs -0 -n 1 -I {} make build
 	
+prepare:
+	npm install
+
 build:
 	cd three.js && make build
 	cd aframe && make build
 
-minify:
+minify: prepare
 	cd three.js && make minify
 	cd aframe && make minify