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

Edit 2: This also contains rewritten cause and reason system at now... I had to include then as I really needed them for certain events.
Why are all of these methods here package-private? They are not accessible from outside of this package.