Simple test functionality

The test function is passed to `exercises/common.js`, e.g.:

``` js
require('../common')({
 test: function(done) {
   var passed = true
   var error = null

    done(error, passed)
 }
})
```

You can use `lib/match-fbo.js` for simple tests: it uses gl-fbo-matching and the FBOs created by gl-compare to check if the output being generated by both shaders is equivalent. lesson-0 is using this as an example, but if you'd like something more specific you can pass in your own function too.

@mikolalysenko let me know what you think :) Thanks!