Fixed #256. Reorganize plugin structure: plugin class now lives in init.py

Some plugins have multiple modules and assets and to make a common structure
we decided to put all plugins under its own package name. The plugin class
itself will be under __init__.py and other helper assets (modules, xml files, etc)
should also be under the plugin folder.

See https://github.com/mozilla/minion-zap-plugin/issues/39