Update links after ownership transfer

Attester now has its own organization, not anymore part of ariatemplates
Expose main module script

To use attester as a module it's necessary to specify a `main` file in the package.json
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.