Implement a new method called find_obfuscated

The main idea is to bring up the same behavior of ActiveRecord::Base#find: raising an ActiveRecord::RecordNotFound exception if the found record does not exist.  In the current implementation of obfuscate this feature is missing, since there is just one way to find a record with its obfuscated id: by using find_by_obfuscated_id.  The later, does not raise any exception because under the hood it uses the ActiveRecord::Base:find_by_id.  So, the main idea is to provide an API with two functions:

1) find_obfuscated --> which behaves as ActiveRecord::Base#find;
2) find_by_obfuscated_id --> which behaves as ActiveRecord::Base#find_by_id