Add NPM scripts to package.json

This serves two purposes:

- The default `npm install && npm test` now suffices to run the test
  suite. This is what Travis and many other systems will do by default
  to build JavaScript projects.

- It removes the need to do $(shell npm bin) hackery in the Makefile.

Makefile

-NPM_BIN := $(shell npm bin)
-
 .PHONY: default
 default: all
 
@@ -15,14 +13,13 @@ clean:
 ## Run test suite
 .PHONY: test
 test: node_modules/.uptodate
-	$(NPM_BIN)/gulp test-app
+	npm test
 
 ################################################################################
 
 build/manifest.json: node_modules/.uptodate
-	$(NPM_BIN)/gulp build-app
+	npm run-script build
 
-node_modules/.uptodate: package.json
-	@echo installing javascript dependencies
-	@$(NPM_BIN)/check-dependencies 2>/dev/null || npm install
+node_modules/.uptodate:
+	npm run-script deps 2>/dev/null || npm install
 	@touch $@

package.json

@@ -134,5 +134,10 @@
     },
     "hammerjs": "Hammer",
     "jquery": "$"
+  },
+  "scripts": {
+    "build": "gulp build-app",
+    "deps": "check-dependencies",
+    "test": "gulp test-app"
   }
 }