Data pipe poc

Simple data-pipes implementation: auxiliary periodic tasks that pour data into buildings, according to configurations stored in the DB.

Still missing:
* More tests of batch handling (API results, data-pipes, buildings)
* Move the web-fetcher into an async service (e.g., under Tornado/Twisted)
* Add cach layer (e.g., Redis) & use to quickly determine building vacancy
* More docstrings
* DB abstraction & injection