Bump tokio from 0.2 to 0.3

[Urhengulas]

Changes

• Cargo.toml
  • bump tokio from 0.2 to 0.3
  • exchange tokio-tls with tokio-native-tls 0.2
• stream:
  • adapt impl AsyncRead for MaybeHttpsStream
  • adapt impl AsyncWrite for MaybeHttpsStream
  • adapt impl Connection for MaybeHttpsStream

Fixes #76

Status

• [x] The examples still need to be adapted, but for that we require hyper to support tokio 0.3 → implemented in https://github.com/hyperium/hyper/pull/2319
  • [ ] hyper release including changes

[godofdream]

Can be closed, as we have an pullrequest for tokio 1.0

[Urhengulas]

Work continued in #79