Adding FlexJS - An Introduction slides to home page.

git-svn-id: https://svn.apache.org/repos/asf/flex/site/trunk@1644728 13f79535-47bb-0310-9956-ffa450edef68
Update thumbnail to be consistent with other thumbnails

git-svn-id: https://svn.apache.org/repos/asf/flex/site/trunk@1644745 13f79535-47bb-0310-9956-ffa450edef68