Add 'mail_from_address' configuration
In environments where there are rules for the email addresses, the "from address" that owncloud uses has to be configurable. This patch adds a new configuration variable 'mail_from_address'. If it is configured, owncloud will use this as the sender of *all* emails. (OwnCloud uses 'sharing-noreply' and 'password-noreply' by default). By using the 'mail_from_address' configuration, only this email address will be used.