upgrade to yui 3.8.0 and use original yui-seed

1. upgrade to yui 3.8.0  to avoid some unexpected bug in sandbox.
2. use original yui-seed (seems more stronger) as yui default seed.
3. According to windjammer team ,qunitjs can't be in whitelist  http://bug.corp.yahoo.com/show_bug.cgi?id=6226130 , so we'd better make other test framework(mocha,jasmine,qunit...) as optional dependencies.