new: add getRootBundleName() to plugin API

Alternative to yahoo/locator#8, needed for @caridy's yahoo/express-yui#16
add api doc comments for getBundle() and getRootBundleName()

- getRootBundleName: for this PR
- getBundle: was missing
Merge pull request #9 from isao/pluginApi

new: add getRootBundleName() to plugin API