Add Material interface

This Material interface is designed to support modded materials. It contains basic functions to get properties of item/block, which are very likely needed by every plugin developer. I decided to use interface because its already used in SpongeAPI. Its is in different package because I think we can use that package later for all Block-specific API interfaces.

Later, we probably need class which contains final static fields for all vanilla materials to make it more helpful for plugin developers. I'll maybe commit also it, if this PR gets accepted. I don't want to do some hours of work if it appears that you're not accepting PRs.
Thoughts on this being getIdentifier() instead, since it's returning the material ID and not a human-readable name?