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
There can only be one cause that caused the event so wouldn't it make sense to type BlockEvent to a BlockCause of sorts? All BlockEvents should have some cause.