Switch from raw couch to nano

This moves us over to nano as our couchDb connector.

Persistence now has 2 nano related functions:

* **nano**: returns a nano instance initialised to point to the configured host
* **nanoDb**: returns a nano.db instance, which is configured to talk to the configured DB (currently, 'whatoss'). This object has been [promised using bluebird](https://github.com/petkaantonov/bluebird/blob/master/API.md#promisepromisifyallobject-target---object). This means nano's built-in functions have promise returning versions which can be accessed by suffixing 'Async' to their names, e.g. `Persistence.nanoDb().getAsync().then(...`