Expose sampler/mipmapping options for entities with material

This [came up on the forum](https://groups.google.com/forum/#!topic/cesium-dev/5usfHVqT9zE). Right now if you're creating an entity with a textured material on it, there's no way to control the sampler that the texture uses, so there's no way to set up mipmapping for it. You can't do this from the primitive API either because `Material.js` does not expose the sampler as an option.

Start by reading Material.js and tracing how textured materials are configured through the entity and primitive APIs. Use the linked forum discussion for context, then verify that sampler and mipmapping options can be controlled for entity materials and that the exposed behavior is covered by the relevant existing tests.

Written by the indexing model from the issue text.