Expose css, link, and head in package. (#154)

* Expose next/css, next/link, and next/head.

This allows test runners like Jest and ESLint's file resolver to
be able to find exported modules within `next`.

Fixes #113

* Removed semicolons for linting and expose modules.

css.js

+module.exports = require('./dist/lib/css')

head.js

+module.exports = require('./dist/lib/head')

link.js

+module.exports = require('./dist/lib/link')

package.json

@@ -6,7 +6,10 @@
   "license": "MIT",
   "repository": "zeit/next.js",
   "files": [
-    "dist"
+    "dist",
+    "link.js",
+    "css.js",
+    "head.js"
   ],
   "bin": {
     "next": "./dist/bin/next"