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.

UPDATE: 
The third commit ([diff 3](https://github.com/emanuele3d/Terasology/compare/emanuele3d:ea3a231...emanuele3d:734898)) addresses the issues discussed so far, namely: private vs public static variables, static variables requiring objects from the registry made non-static, a few small changes here and there, and a slightly different handling of the camera position in the comparators, awaiting for a more complete solution in following PRs.
I tried unsuccessfully to use a member variable in the comparators to hold the camera. The problem is that if the camera changes, the reference held by the comparators need to be updated and all the collections taking advantage of them need to be invalidated and refreshed. 

I did find room for a small performance improvement, halving the number of times the camera is obtained through the registry, but I only added a TODO to eventually solve the bigger issue: ideally a change of camera should be checked only once per frame and the collections using the comparators should be updated only as necessary.