added contributing information

Signed-off-by: Kai Kreuzer <kai@openhab.org> (github: @kaikreuzer)
fixed potential NPE

Signed-off-by: Kai Kreuzer <kai@openhab.org> (github: @kaikreuzer)
made OSGi HTTP service optional

Signed-off-by: Kai Kreuzer <kai@openhab.org> (github: @kaikreuzer)
Fixed NPE if listeners are registered before startup

Signed-off-by: Kai Kreuzer <kai@openhab.org> (github: @kaikreuzer)