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
Is it possible in the case of water air water to have a method that would return all the blocks used to spread the water, not just the one causing the event?