Use tweens instead of wrapping the app

Fixes #3.

I already have changes ready for morepath: https://github.com/seantis/morepath/compare/morepath:master...master
And for morepath_static: https://github.com/seantis/morepath_static/compare/morepath:master...master

I figured they would come last.

Also, this change depends on morepath 0.8 and bowserstatic 0.6. With the idea that more.static should get released after morepath and bowerstatic.

Let me know what you think.