Add support for Tokio (async) (#10)

Fixes #6

nut-client/examples/async.rs

@@ -0,0 +1,42 @@
+use std::env;
+
+use nut_client::tokio::Connection;
+use nut_client::{Auth, ConfigBuilder};
+use std::convert::TryInto;
+
+#[tokio::main]
+async fn main() -> nut_client::Result<()> {
+    let host = env::var("NUT_HOST").unwrap_or_else(|_| "localhost".into());
+    let port = env::var("NUT_PORT")
+        .ok()
+        .map(|s| s.parse::<u16>().ok())
+        .flatten()
+        .unwrap_or(3493);
+
+    let username = env::var("NUT_USER").ok();
+    let password = env::var("NUT_PASSWORD").ok();
+    let auth = username.map(|username| Auth::new(username, password));
+
+    let config = ConfigBuilder::new()
+        .with_host((host, port).try_into().unwrap_or_default())
+        .with_auth(auth)
+        .with_debug(false) // Turn this on for debugging network chatter
+        .build();
+
+    let mut conn = Connection::new(&config).await?;
+
+    // Print a list of all UPS devices
+    println!("Connected UPS devices:");
+    for (name, description) in conn.list_ups().await? {
+        println!("\t- Name: {}", name);
+        println!("\t  Description: {}", description);
+
+        // List UPS variables (key = val)
+        println!("\t  Variables:");
+        for var in conn.list_vars(&name).await? {
+            println!("\t\t- {}", var);
+        }
+    }
+
+    Ok(())
+}