newsletter: implemented fallback for Pimcore_Tool::getHostUrl() since this isn't always always available (if main domain isn't set in system settings) + improved getHostUrl()

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@4412 1f8fe7d8-47f0-464c-8d0a-336f4953ab05