fix page.injectFile and add test. (#52)

This line within `injectFile` wasn't doing much of anything: 

```js
let expression = fs.readFile(filePath, 'utf8', (err, data) => callback({err, data}));
```

* That's fixed.
* A path error in examples/features.js is fixed.
* Test added for injectFile.

test/assets/injectedfile.js

@@ -0,0 +1 @@
+__injected = 42;