Externalize asset system + improve docs

[Cervator]

Edit 2015-11-14: This is mostly complete (gestalt-asset has been extracted and stabilized), leaving the issue open for a bit till we get a chance to review / improve docs.

@immortius has mentioned wanted to pull out our engine's asset system to a gestalt-asset or so library. That's probably also a good time to overhaul the documentation, first for the system in its own repo wiki and our docs here for Terasology

I was looking to update https://github.com/MovingBlocks/Terasology/wiki/Asset-System as part of merging a new asset type but it is woefully out of date and mostly just a clone of AssetType.java's list. Ideally we'd put a more informative page together as part of the modding guide for Terasology's used types that is more resilient to going out of date. That could be done by scanning AssetType.java (or any tweaked setup with gestalt-asset torn out) and comparing it to the wiki page as part of the build in Jenkins.

Related: #1217, #813 (probably?), #763 (maybe), #335 (perhaps obsolete/fixed at this point)

[shartte]

What is the benefit of externalizing the asset system?

[Cervator]

Reusability (everybody loves libraries!), clarity, encapsulation, shrinking engine further, fewer chances for code/git conflicts. It isn't very critical, but nice :-)

[msteiger]

It would also be great to have this for the MapViewer project. Currently, it fires up TS in headless mode just to access simple assets such as the name list for NameGenerator.

[immortius]

One particular driver is opening up NUI for use outside Terasology.

[Cervator]

Closing as complete. Was actually done some time back but this was left open for documentation. Between https://github.com/MovingBlocks/gestalt/wiki/Gestalt%20Asset%20Core and https://github.com/Terasology/TutorialAssetSystem that should be covered!