merged volatile service PR

Signed-off-by: Kai Kreuzer <kai@openhab.org>
incorporated Kai's review comments
Added typed return values and tests

Signed-off-by: Kai Kreuzer <kai@openhab.org> (github: @kaikreuzer)
Signed-off-by: Kai Kreuzer <kai@openhab.org>
Fixed test execution during Maven build

Signed-off-by: Kai Kreuzer <kai@openhab.org>
Merge branch 'master' of https://github.com/qivicon/smarthome

Signed-off-by: Kai Kreuzer <kai@openhab.org>