Async API

[bbb651]

I'm building a logcat viewer with iced, and it would be nice to use this crate instead of adb logcat.
Iced make heavy use of async and even with spawn_blocking I couldn't manage to integrate it without blocking the executor forever, it would be really nice if this crate had a async variant of the api.
I think it's also a requirement for #121 as blocking on web is disallowed?

P.S. ADBServerDevice::get_logs has the the following signature:

pub fn get_logs<W: Write>(&mut self, output: W) -> Result<()>;

Is there a reason it take a Write instead of returning a Read? E.g.:

pub fn get_logs(&mut self) -> Result<LogReader>;
impl Read for LogReader;

This allows directly reading from it, you could always use std::io::copy if you want to write it to an impl Write.