Add Async/Tokio support

[gdetal]

This PR implements #5 by:

• upgrading the project to rust 2018
• integrating latest tokio library
• adding a new feature ("async") to the project

[meh]

This is lovely! Thank you!

Just one request, could you reword the commits to be chore: update to edition 2018 and feat: add async/tokio support please?

[gdetal]

Sure !