allowing app.yui._defaultBundle to be used as the fallback if the user does not default defaultBundle or bundle in app.js
identifying if the bundle is root, then using it as _defaultBundle
removing the defaultBundle since we can know figure that out from locator
bumping version to 0.5.4 where defaultBundle is not required, but optional
adjusting the code to use api.getRootBundleName() instead of bundle.isRoot since that is more appropiate. updating tests
adjusting docs to reflect that defaultBundle is now optional
Merge pull request #16 from caridy/master

defaultBundle is not longer required, but optional