Add PAM authentification to Deluged

[v-for-vandal]

This patch is similar to the  659. It adds authentication via PAM as option. Idea is: On the beginig, AuthManager? determine the available to the daemon auth methods( i.e. check that there is PAM Python module ) and create a list of the available methods. Then it read the list of the desired methods from the config variable "auth_methods" ( defaulted to 'file' ) and remove from this list unavailable methods. When auth request is recived, AuthManager? call necessary functions in specified order. If any of them success, then authentication is successfull, otherwise a BadLoginError? is raised.

Flaws: Now there is no way to specify what exactly was wrong in authentication for each function.

[v-for-vandal]

Patch that implement changes