GitLab

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 and #19

See merge request !24