Gzip

This adds gzip decompression support.

You probably don't want to include as-is; I'm using Rust 0.11.0. I'm also using Meson to compile (Cargo didn't work; maybe the old version), which required renaming a couple of files (Meson's Rust support is in its infancy).

Code review is also welcome. I've tested against a few gzip examples (included).