Remove log peer dependency

[yoshuawuyts]

Currently log is a peer dependency to provide access to the logging macros (such as info!() and debug!()).

We should find a way to re-export the macros from log through this crate. This is possibly blocked on Rust 2018.

From Discord:

i think in rust 2018 you can pub use macros::your_macro

[killercup]

also from discord :D