Make `gpu_cache` optional

Created by: lo48576

I think gpu_cache module and related features should be optional, so how is it to use "feature" feature? This change will help solve also #12, by not turning on the gpu_cache feature on Redox os. The feature is (currently) disabled by default, but can be enabled by building with cargo build --features="gpu_cache", and gpu_cache example can run by cargo run --example=gpu_cache --features="gpu_cache". (See http://doc.crates.io/manifest.html#the-features-section )