Checked plone.openid with pep8 and added option to whitelist OpenID providers.

* Checked and fixed every .py file with pep8.
* Added allowed_openid_providers property to plone.openid plugin. With this administrators can set whitelist of openid providers which are allowed to proceed from extractCredentials method. This is very useful for sites which want to control where OpenID users come from.
* Added tests for above.