Implement API.find method

Query the Google Books API to find a book by its unique VolumeID
Simplify query method with URI.encode_www_form
Use autoload require only when module is accessed
Remove unused cgi dependency
Fix typo find method return API::Book instance
Use HTTParty query tricks and handle API errors
Merge pull request #3 from madwork/find-by-id

Implement API.find method to find a book by its unique VolumeID