RenderableWorld changes

This PR follows #1456 and focuses only on the new class, **RenderableWorldImpl**.

The first commit ([diff 1](https://github.com/emanuele3d/Terasology/compare/MovingBlocks:develop...emanuele3d:c68febe)) mostly just renames variables and methods to improve readability.
The second commit ([diff 2](https://github.com/emanuele3d/Terasology/compare/emanuele3d:c68febe...emanuele3d:ea3a231)) does a number of things:
* extracts variable instantiation from loops when appropriate
* reviews and refactors some of the logic where appropriate (*reviewers needed!*)
* compacts some methods
* removes some duplication

The second commit also augments the RenderableChunk interface and the ChunkImpl class by adding methods hasMesh() and hasPendingMesh(). Both have been put in place to increase readibility, as they replace statements such as "if(chunk.getMesh() != null)" with "if(chunk.hasMesh())"; 

RenderableWorldImpl will be further modified when I start working on WorldRendererLwjgl.
Notice that this PR comes with some line-specific questions.
I think I can answer that - chunk meshes are generated in a separate thread. After the thread finishes generating it, it is placed in pendingMesh to be picked up by the main thread.