Fix subscribe target issues.

subscribe used for some global UI events, but sometimes we do have use cases that we need to subscribe to certain target. Although subscribing different targets cannot share same event, but it still can get benefits from throttling.

Right now subscribing to specific target is still under documented. We will document it when it is more stable.

Thanks @ebertb.

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.