add 'getUserBySearch' method

Added a method to search for a user by any criteria.

https://docs.auth0.com/api#get--api-users-search--criteria-
Added 'getUserBySearch' method

https://docs.auth0.com/api#get--api-users-search--criteria-
Merge pull request #14 from rorygarand/master

Added user search method named `getUserBySearch`