Use authentication_keys for resource lookup instead of static email field

[zhaknafein]

The Devise gem allows to use different authentication_keys instead of the "email" field. This change (zhaknafein/devise-api@bb8057994632c93db74ab5ba006e5b81f56479b3) will make devise-api follow the same configuration as Devise. Sorry for the missing translations and tests, i accept suggestions on how to replace the "email" word everywhere :)