* Allows radius to be a symbol representing a column in DB

* Proposed changes by @alexreisner:

- Keeping branching condition consistent
- Not overwrite the method argument: ‘radius’
- Not assigning two variables in a single line, and
- Add code comments where needed

By “proposed” I meant, I copy/paste them from his comment. But we are
in agreement.

* Fixes broken test in PR

To run a single file, use:

  ruby -I test <filename>

Functionality to pass optional :min_radius value to 'near' searches. Only works with ActiveRecord and not Sqlite.

Should close #374.

Should resolve half of #374.