getUserBySearch

I added a 'getUserBySearch' method to the library.  This allows you to get a User via any search criteria that's allowed by the '/users?search={criteria}' API call.

To do this I directly copied the code for 'getUser' and modified the two lines that have the variable 'searchCriteria'.

https://docs.auth0.com/api#get--api-users-search--criteria-