Add postcode field to search

Depends on [this PR](https://github.com/alphagov/business-support-api/pull/23) to handle partial postcodes.

Adds a 'Business postcode' field to the BSF search.

![screenshot - 170914 - 10 53 29](https://cloud.githubusercontent.com/assets/93511/4301558/85c142ac-3e50-11e4-8abf-2225454f2c38.png)