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
This is a style question. In a fluid spread, is the origin of the event the block that the fluid spread to or the source fluid? I believe that the origin is the source fluid as you do, so then getDestination() or something. However, getSource should still be there, I think, just for consistency and ease-of-use, for just calling getBlock().