Merged with gae-init/gae-init-auth: refactoring signin stuff.
Merge pull request #225 from gmist/simplify_signin

Refactoring signin stuff
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.