Feature expose all models

I've branched off #9 in order to flesh out a basic working version of the API.

This now has routes for all levels of content, as well as simple "list" interfaces to get lists of series, releases and datasets. I've not exposed observations as there's a lot of these per dataset, will need to add paging.

This has been tested against my local mongo instance which has a copy of the data loaded from `ons-poc-data`.

There's plenty of room for improvement, but there should be enough here to allow us to start iterating on the UI much faster now.