plugin: added ORO_GLOBAL_SERVICE_NAMED_PLUGIN for service plugins that want to install a global service

Signed-off-by: Johannes Meyer <johannes@intermodalics.eu>
tests: extended plugins_test to include global service plugins

Signed-off-by: Johannes Meyer <johannes@intermodalics.eu>
Merge pull request #62 from meyerj/global-service-named-plugin

Added ORO_GLOBAL_SERVICE_PLUGIN macro to ServicePlugin.hpp