Replacing Flask-OAuth with Flask-OAuthlib
Merge pull request #218 from mdxs/use_flask_oauthlib

Replacing Flask-OAuth with Flask-OAuthlib
Used Flask-RESTful for API.
Fix fields type.
Improved API errors handler.
Added request.query_string value to JSON link.
Added 'check_unique_email' field to config API.
Sorted properties in CONFIG_FIELDS.
Added recaptcha fields.
Minor fix.
Sorted User fields.