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.
The concepts mixed around in this are:
* Whether a chunk is surrounded by loaded chunks - this is required for chunk tessellation (edges can only be tessellated if you know what is adjacent to them). This is part of the valid for render check.
* Whether a chunk has completed generation and light merge - this isn't as big a deal anymore since the chunk generation was simplified, but there is still a gap between when a chunk is added to the chunk provider, and when it has been merged with the surrounding lighting. There is a gap between this events, and the mesh shouldn't be generated before it is complete. This is what it means for a chunk to be 'ready'. A ready chunk never ceases to be ready unless it is unloaded and then reloaded.
* Dirtiness is whether a chunk has changed, or an adjacent chunk has changed, such that the chunk's mesh needs to be regenerated. Adjacent changes can cause this if they reveal part of the chunk's blocks that were previously hidden (or hides part of the chunk's blocks).