Expose listen

listen is a simple and cross-browser library to do the same thing as addEventListener does. Although, there are tons of libraries have it, but it is convenient to expose it if you don't want to require other libraries.

v1.0.0: Merge pull request #20 from yahoo/v1

Some breaking changes:

1. Remove unsubscribe standalone function. Please use sub = subscribe('scroll', func); sub.unsubscribe()
2. Remove viewportchange virtual event.