Resolve "Create the plugin_event"

• For list and metric, filtering conditions can be written using the web2py syntax for database query. Therefore, it can deal with foreign tables of the history table as well as with the history.data field.
• The smart_query is still available but deprecated since it can not handle foreign fields.
• Factorize the application around the plugin_event.
• Closes #16 (closed) and #19 (closed)