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.
added function documentation
updated config.sample.php with mail_from_address parameter
Merge pull request #6930 from jcfischer/master

Add 'mail_from_address' configuration