Add GetEventSender function, Add Node::GetChild function.

git-svn-id: http://urho3d.googlecode.com/svn/trunk@2515 3bd170d8-ddcf-f82c-f583-7a401ccf5608
Port SoundEffect sample to Lua.

git-svn-id: http://urho3d.googlecode.com/svn/trunk@2516 3bd170d8-ddcf-f82c-f583-7a401ccf5608