Added property to define an optional image host to prevent a certain class of Cross-Site-Scripting attacks. The property allows to specify a separate domain that is used in constructing image URLs.

File 'imap.properties' is enhanced by property com.openexchange.mail.imageHost. Default is empty; no image host used.

Exemplary setup:
- Artificially add a host name to /etc/hosts:
127.0.0.1 imageserver.open-xchange.com
- Enable the com.openexchange.mail.imageHost property in mail.properties:
com.openexchange.mail.imageHost=http://imageserver.open-xchange.com
- Check a mail with an inline image