slog v2

Changes overview

Core crate:

• Refactored key-value pairs traits and structures
• Renamed some types
• Support chaining of OwnedKVLists
• Added associated Ok type to Drain
• Refactored Record to optimize performance on async operations
• Minimal rustc version required: 1.15.0
• DrainExt has been removed and utility methods moved directly to Drain

Other crates

• slog-extra has been renamed to slog-async since that's the only functionality it contained.
• slog-stream is obsoleted and won't be used in slog v2 at all. It was a wrong abstraction. Decorators functionality will be moved directly to slog-term.
• slog-term will provide open io traits to allow multiple terminal / file writing backends.