Rewrite search logic to be driven by elasticsearch

[benglass]

Rewrote the search logic to use the ElasticSearch engine.

Reasons:

• Speed: It's faster especially for geodistance calculations than using native MySQL with joins
• Flexibility: Established platform allows for complex queries relatively easily (for #22)
• Robustness: Out of the box support for geolocation within polygons, which allows us to achieve #16 relatively easily (and support arbitrary polygons)

Resolves #16