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
My first thought when I saw this was "Use `Optional`" But, I think this needs a little more thought: `@Nullable` may be appropriate.