Add support for new Description tag in site and group configs

This is needed to support site and group descriptions with newline characters in an upcoming Nexpose release. This change should be backwards-compatible, though, and can probably be merged in at any time. The `<Description>` tag is ignored by current Nexpose versions and is optional for future versions.
Should fix this XML generate from String manipulation to REXML, adding a `as_xml` method, then the `to_xml` just calls `as_xml.to_s`.