Email contact form settings to send FROM site email adresse (avoids problems with spam filters)

the email contact form is using the email-adress from the from to send emails to the mail adress specified in the email field.
This causes problems with spam configurations on various servers:

• Some web hoster's configurations don't allow sending emails that aren't hosted on the same server ("Unable to send e-mail. Please contact the site administrator if the problem persists" error message would get thrown)
• Spam filters classify emails as spam for the same reason

I attached a little patch (that needs review!) providing:

• a little checkbox on the settings page to activate sending emails with the site's main email address
• The email adress of the sender (from the email contact form) is included into the emails body
• If activated a little message is included on the bottom of the email message advising that users shouldn't reply directly

check it :)