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
A lot of things can cause a block event. For example, an enderman can place a block. I don't think causes should only be blocks.