OpenSocial Gadget Changes

The OpenSocial gadget tutorials have been updated for a more general overview of features, instead of a sample app focus. The *Understanding PubSub Communication* tutorial was removed because it was more of a proof/interactive tutorial than a substance tutorial.

https://issues.liferay.com/browse/LRDOCS-1020
https://issues.liferay.com/browse/LRDOCS-1021