Getting ready for rust 1.0

See #11

As discussed, I've removed `util.rs` and `time.rs`, I've moved the contents of `midi.rs` into `lib.rs`. Additionally I've also split the new `lib.rs` into `ffi.rs` and `tests.rs` to make it more manageable.

I've removed a bunch of warnings too, but not all of them yet.

Are there any other changes you'd like me to make?