Export the find function directly

If the output of a module is only a function, it's idiomatic in node to export it directly using `module.exports` rather than hanging it off of an otherwise-empty object literal.

Updated code, tests, and README.

Please forgive the opinionated pull-request :)

Reference: http://stackoverflow.com/questions/17936515/best-practice-for-structuring-libraries-to-be-required-in-node-js though there are others if you google "substack pattern"