World/Chunk load/unload events

Adds abstract classes for `World`/`Chunk` load and unload events.
Adding BlockEvent abstract class

Adding a base `BlockEvent` interface.
Add Contribution Guidlines link to README
Merge pull request #20 from prism/events-world

World/Chunk load/unload events
Merge pull request #34 from prism/events-block

Adding BlockEvent abstract class
Merge pull request #39 from simon816/readme-guideline-link

Add Contribution Guidlines link to README
Merge pull request #1 from SpongePowered/master

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