Allow symbol to mean radius comes from a db column

Hi Alex, please take a look at this change. There's may be a cleaner way to do it, but this was quick-and-dirty and does what I needed.

The use-case this new feature supports is for the case when each record has its own radius (e.g., a delivery radius for a store), and you as a customer want to see if you qualify. If the radius you pass in to .near(...) is a symbol, e.g., :delivery_radius_miles, then that text gets passed through to the query as a column name rather than getting cast in ruby to a float and used in app-level calculations. There is a side-effect in how I implemented it; even if you pass a numeric value, the calculation is still done in the database layer.