Plugin for test pages

After the refactoring this is the first pull request that allows using plugins, so far only to modify the content of a test page.

This is achieved by exposing some middlewares and adding the modules

- `plugins` where you can require a plugin from disk or npm
- `testPage` where you can add element into a test page

For testing purposes, this pull request introduces also an `attester.dispose` to cleanly dispose all objects created.