Release of 0.13.0

[arlyon]

The library is in a good place. Here are some things we'd like to get done before 0.13.0:

• port the tokio and hyper code to support 1.0 (Support tokio 1.0 #1)
• review the API and make sure we aren't missing things from the old API
• automatically update the openapi spec
• remove all unwraps from the code (Remove uses of unwrap #20)

[kennetpostigo]

One more thing is probably updating the docs, i'm currently referencing the stripe-rs one.

[arlyon]

Docs should be up as of last week, still need to iron out the client documentation but the API is covered at least! :)

[kennetpostigo]

@arlyon thank you sooo much! I've been using the async-stripe so far and it has gone great!

[arlyon]

For an update here, I would like to flesh out the SetupIntent api a little then I will think about (finally) releasing 0.13.0. I just dropped 0.13.0-rc3 which reworks the error code a bit and should be the last breaking change.