Support Beta 1.8 (Closes #67)

[KaiSandstrom]

These changes add support for Beta 1.8.

Differences in biome generation between Beta 1.8 and Release 1.0 include:

• Beta 1.8 layer stack starts at a 1:8192 scale instead of 1:4096.
• mapLand (GenLayerAddIsland) handles RNG differently in Beta 1.8, replacing v11's biome when the high value is returned rather than the low value, giving different results even for the same seed/coords/surroundings.
• Ice plains and mushroom islands were not yet added.
• A bug in the AddIsland layer (when used once at 1:32) causes small patches of ocean to generate at the edges of plains biomes and small patches of plains to generate along coastlines.

Aside from changing getStructureConfig to show no nether fortresses in Beta 1.8, no changes have yet been made to the structure finders. Village positions are tested and correct. Mineshaft and stronghold finders are untested, will do further work in the future.

[Cubitect]

Looks good, thank you. I might take this opportunity to also refactor some of the MC version dependencies in the project to make it easier to potentially support other early and minor releases in the future.

[KaiSandstrom]

Oh, awesome! I've been working on some preliminary tests for pre-B1.8 biome tests actually, so I may make another PR with that eventually. Thanks!