Create plugin for OAuth2 authentication

Create a plugin for OAuth2 authentication which can be used by the module wsclient.

The attached patch implements this plugin.
It assumes that this patch has already been applied:
https://drupal.org/files/http_client-2042205-create_basic_http_auth_plug...

This oauth2 plugin will be used by wsclient, and this patch on wsclient needs to be applied,
so that it can be used: https://drupal.org/files/issues/wsclient-2138617-oauth2_support.patch

This plugin depends also on the module oauth2_client, which can be downloaded from:
https://github.com/dashohoxha/oauth2_client
The same module is at drupal.org as a sandbox project: https://drupal.org/sandbox/dashohoxha/2133827
A request has been made for reviewing and accepting this module as a full project, see:
https://drupal.org/node/2138549
If you can, please help to review and support it.