add header decode and tests, make cart public so it's testable
Merge branch 'master' of github.com:colemickens/rustnes

Conflicts:
	src/cart.rs
git rid of byte and word types
Merge branch 'master' of https://github.com/colemickens/rustnes into cole-master

Conflicts:
	src/cart.rs
	src/lib.rs
Merge branch 'cole-master'