Tokio v0.2.0

A major breaking change. Most implementation and APIs have changed one way or
another. This changelog entry contains a highlight

Changed

• APIs are updated to use async / await.
• most tokio-* crates are collapsed into this crate.
• Scheduler is rewritten.
• tokio::spawn returns a JoinHandle.
• A single I/O / timer is used per runtime.
• I/O driver uses a concurrent slab for allocating state.
• components are made available via feature flag.
• Use bytes 0.5
• tokio::codec is moved to tokio-util.

Removed

• Standalone timer and net drivers are removed, use Runtime instead
• current_thread runtime is removed, use tokio::runtime::Runtime with
  basic_scheduler instead.