Collect nameko events for insertion into staging table

This PR is a partial implementation of the remaining piece of the pipeline between platform and data warehouse. It should not be deployed as-is, however, as it cannot yet be fully completed. That is, it cannot be built in a generic form until *nameko* is able to offer a full solution to handling multiple events through a single handler. This will need to be a programmatic version of the existing `event_handler` decorator.

In the meantime, the process is illustrated here by a class containing three hard-coded, decorated event handlers, each of which put a record into the staging table. Note that this process assumes the staging table has already been built.

Overall then, the final process should be able to dynamically generate event handlers based on a set of event names taken from a list of active fact definitions. These active facts will likely be defined within *settings* or a fixed-name index file. These handlers will all perform a similar action to the hard-coded ones here, varying only in the name of the event written (e.g. 'service_name.event_name') and the content of the JSON data.