Really fix avatar and prop resizing issues (#36 and #38)

Props and avatars now display as close as their actual sizes as possible. Fix these 2 issues: https://github.com/AUTUpStageTeam/UpStage2013/issues/36 and https://github.com/AUTUpStageTeam/UpStage2013/pull/38

Since stage assets are loaded into client.swf, which has a fix size of 320x200 and is stretched to full screen on browsers (and each computer has different screen sizes), it is impossible to display props and avatars in their actual sizes on every screen.

In this fix, I use 1280x800 screen size as the reference size, so the closer screen size is to 1280x800, the more accurate the assets size on stage will be.

Width and height attributes (of swf assets) are also added to avatars, props, and backdrops xml files.

Ing