Add option to limit search result by distance

If you have many stores, e.g. all over US and user searches for New
York, allow the website to show only stores within a certain distance,
e.g. in 50 miles radius of New York.

Feature is turned off by default (the current stores setting) and can be
turn on by setting STORES_MAX_SEARCH_DISTANCE for central limit or can
be modified by overriding get_max_distance() for dynamic values, e.g.
changing the distance for certain queries.
Merge pull request #68 from tangentlabs/feature/distance_filtering

Add option to limit search result by distance