Adds packing list

Sorry this got so large. In summary:

* Adds packing list read and update views
* Packing aggregate is implemented as a Couch "join" (using complex keys and view collation tricks, see "[Joins with views](http://docs.couchdb.org/en/1.6.1/couchapp/views/joins.html))
* Manages Couch views in the repo
* Adds local fixtures (hoped to generate these via [data_model](https://github.com/eHealthAfrica/data_model), but time didn't permit)
* However, pushed schemas back up to data_model
* Adds specs
* Adds lightweight Couch service (penned to be replaced with Pouch)

Usage:

```
for i in DELETE PUT; do curl -X $i http://localhost:5984/deliveries/; done && gulp views fixtures-local serve
```