Add basic block events

As the block API got merged recently, I made some basic block events. Even if this PR won't get merged, it would not hurt to discuss about block events.

Edit: There is also issue http://issues.spongepowered.org/youtrack/issue/SPONGE-62
If we're going to have this then `isUnknown` isn't necessary, because devs will be doing `instanceof` checks anyways.