Only one instance of the bus is available. Even if Singletons are bad, it's perfect for an event bus. No one have to pass the bus as an param with the constructor.
Added comments to each method

Like Java-Doc a programmer can see the method description in a good IDE
Added support for more than one bus.

There can be more than one bus instances, but it is extremly easy to get
a specific instance.
Merge pull request #2 from ibiBgOR/master

Only one instance of the bus is available.