Add support for RESTful JSON APIs access tokens and OAuth2.0 For Login

This commit adds a new submodule feature 'access_token', it is intended
for RESTful JSON APIs, it allows rich client-side applications to use
access tokens to communicate with the backend server.

It also adds support for OAuth 2.0 For Login - Implicit Grant Flow -.
Fix, do not hit the database when access_token request argument is blank
Fix, curl example in README file was missing a comma
add xing provider
Merge pull request #419 from rweng/xing-provider

add xing provider
merged
fix merge