migrate to async API

[rursprung]

see my comment on #8 and #11: you may want to migrate to an async API

most of the rust ecosystem - at the very least anything doing I/O - seems to have gone down that route, and for good reasons.

after #11 this is very easy for the client part. for the server you'll have to replace rouille with an async HTTP server, e.g. rocket.
this would be a breaking change for clients if you no longer provide a blocking API, but i don't think that it's worth it for you to maintain both an async and a blocking API.