Add example for packet sending support

This way, the one-way-link example (basically) works!

examples/lora_packet_tx.rs

@@ -0,0 +1,37 @@
+use rn2903::{bytes_to_string, Rn2903};
+use std::env::args;
+use std::process::exit;
+use std::thread;
+use std::time::Duration;
+use std::time::{SystemTime, UNIX_EPOCH};
+
+fn main() {
+    let args: Vec<_> = args().collect();
+    if args.len() <= 1 {
+        eprintln!("rn2903_lora_packet_tx <serial port>");
+        eprintln!("\tSend LoRa packets with the current UNIX time every 5 seconds.");
+        exit(1);
+    }
+
+    let mut txvr = Rn2903::new_at(&args[1]).expect("Could not open device. Error");
+    println!(
+        "Successfully connected. Version: {}",
+        txvr.system_version()
+            .expect("Could not read from device. Error:")
+    );
+ 
+    txvr.mac_pause().unwrap();
+    txvr.transact(b"sys set pindig GPIO10 0").unwrap();
+    loop {
+        txvr.transact(b"sys set pindig GPIO10 1").unwrap();
+        let now = SystemTime::now();
+        let since_the_epoch = now 
+            .duration_since(UNIX_EPOCH)
+            .expect("Time went backwards")
+            .as_millis();
+        println!("sending: {}", since_the_epoch);
+        txvr.radio_tx(format!("{}", since_the_epoch));
+        txvr.transact(b"sys set pindig GPIO10 0").unwrap();
+        thread::sleep(Duration::from_millis(100));
+    }
+}