Add a config variable for search radius
Add a base model for search forms

This implements the ActiveModel interface and backs
a search form, allowing you to use ActiveModel::Validations
and friends to process and perform searches.

This is in app/models so that themes can then override it
as necessary by inheriting from it and filling in the
(currently blank) methods as required.
Refactor Hampshire's search controller to use a model-backed form

This moves all the logic for processing search input and
computing the search results into a model which inherits
from ApplicationSearch, using hampshire specific logic
to do so.
Update Hampshire search views to match new form object
Update Hampshire feature specs to match new search form

I've moved each VCR episode into an individual file for
each example, as that seems to be more how VCR expects
you to use it, and it makes it easier to keep track of
which examples need updating if something is refactored.
Merge branch 'issues/81-search-refactor' into hampshire