make npm scripts work under Windows

* Use mkdirp from npm as a cross-platform solution for mkdir -p
* fix quotes of argument to uglify
* not sure whether specifying the path node_modules/mocha/bin/ is an
  antipattern (some comments by npm authors claim so) but istanbul does
  not seem to take the PATH into account

package.json

@@ -25,11 +25,11 @@
     "async"
   ],
   "scripts": {
-    "build-dist": "mkdir -p dist && rollup src/main.js | babel -m umd --module-id creed -o dist/creed.js --loose es6.classes",
-    "build": "npm run build-dist && uglifyjs -c 'warnings=false' -m -o dist/creed.min.js -- dist/creed.js",
+    "build-dist": "mkdirp dist && rollup src/main.js | babel -m umd --module-id creed -o dist/creed.js --loose es6.classes",
+    "build": "npm run build-dist && uglifyjs -c \"warnings=false\" -m -o dist/creed.min.js -- dist/creed.js",
     "preversion": "npm run build",
     "test-aplus": "promises-aplus-tests test/aplus.js --reporter dot",
-    "test-main": "istanbul cover --root src _mocha -- --reporter dot",
+    "test-main": "istanbul cover --root src node_modules/mocha/bin/_mocha -- --reporter dot",
     "check-coverage": "istanbul check-coverage --statements 100 --branches 100 --lines 100 --functions 100 coverage/coverage*.json",
     "lint": "jsinspect src && eslint src",
     "test": "npm run lint && npm run build && npm run test-main && npm run check-coverage && npm run test-aplus"
@@ -45,6 +45,7 @@
     "eslint-plugin-standard": "^1.3.2",
     "istanbul": "^0.4.0",
     "jsinspect": "^0.7.0",
+    "mkdirp": "^0.5.1",
     "mocha": "^2.3.0",
     "promises-aplus-tests": "^2.1.1",
     "rollup": "^0.26.2",