Use slog as a logging subsystem

[mrjoe7]

This PR replaces log crate by the slog crate. All code was written with a backward compatibility in mind. When slog::Logger is not passed to actor system via the SystemBuilder, then behavior and log messages format stays the same as was before this PR.

Example of how slog can be passed to riker.

#[macro_use]
extern crate slog;
extern crate sloggers;

use sloggers::Build;
use sloggers::terminal::{TerminalLoggerBuilder, Destination};
use sloggers::types::Severity;

let mut builder = TerminalLoggerBuilder::new();
builder.level(Severity::Debug);
builder.destination(Destination::Stderr);

let logger = builder.build().unwrap();
let sys = SystemBuilder::new()
    .log(logger)
    .create().unwrap();

[olexiyb]

merged into actors.rs