Final touches

[dpc]

• flatten drain and logger module into root namespace
• RecordInfo -> Record
• Node -> List
• move atomic switcher to separate crate, to remove crossbeam dependency
• IntoMsg -> Message trait
  • Use write_to where possible for speedup?
• Remove rustc_serialize dep
• Write drain to old-style logging, so libraries can take Option<Logger> and use drain to old-style logging in case their user doesn't want to use slog.
  • Document it somewhere
• More documentation!
• Move decorators out of the slog create? This way they can be improved after 1.0 release, and eventually even moved back to main crate, without breaking compatibility.
• Lazy values that can actually write to io::Write? Only then allocation can be completely avoided.