Added emit method and modified jsdoc

I have added an 'emit' method because I thought it to be the easiest way to use `Hey` to have a global event communication in my app.

I have also improved the JSdoc to make it possible to use code hinting (at least in IntelliJ) and used more standard terminology in the descriptions.
Never increase the version number in a PR. The maintainer will decide when to do that, and usually as a separate commit :)