Twitter gadget URL is now updated

Dennis notified me that his Twitter gadget XML code can now be accessed through Github. The XML is up-to-date and the URL listed in the User Guide is now referenced from Dennis' new repo on Github.

Also, now that the OpenSocial Gadget Editor is viewable in Portal, I updated the image in chapter 1.

https://issues.liferay.com/browse/LRDOCS-754