Removed fallback "From:" email addess from getattr().

Also updated docs to make explicit that ``DEFAULT_FROM_ADDRESS`` is the one always used to send the emails.

Currently it seems we are ok with always using ``DEFAULT_FROM_ADDRESS``.
The old behaviour (before fixing #27), was to use the address supplied by the user to set the "From:" header.

We will need to add a new setting in case we want to be able to choose between the current and the old  behaviour. Anyone who needs this should open a new issue.