Use external identifier for authentication (example: log on using an email address)

This feature should let the authentication mechanism work with an external identifier that gets converted to an XWiki one. Example: it should be possible to log on using an email address that is checked against an external authentification service (a web service for instance), and that is then converted to an XWiki identifier.

Here's a possible implementation resulting from a discussion with Ludovic:

• email pass
• MyAuthService.checkAuth
• MyFormAuthenticator
• MyAuthService.authenticate: detect, email only-> check email/password against the web service authenticator, retrieve the XWiki ID, say ok, concatenate ID+EMAIL in the Principal
• MyFormAuthenticator: saves the Principal in the cookie
• MyAuthService returns the XWiki ID encapsulated in a XWikiUser