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.
If this should be "never, ever null", why are you not doing any null checks when constructing a cause? Something like `Preconditions.checkNotNull(reason)` should help with this.