Some small fixes and compile flaggery

src/ijvm/machine.rs

@@ -66,7 +66,7 @@ impl Machine {
                 }
             },
             Ok(Operation::Invalid(a)) => {
-                println!("OP: {}", a);
+                println!("UNDEFINED OP: 0x{:X}", a);
                 Err("Invalid op")
             },
             Err(str) => Err(str)

src/ijvm/netstack.rs

@@ -1,7 +1,11 @@
-use ijvm::Result;
-use std::io::{Read, Write};
-use std::net::{TcpListener, TcpStream};
+use std::net::TcpStream;
 
+#[cfg(feature = "bonus:network")]
+use ijvm::Result;
+#[cfg(feature = "bonus:network")]
+use std::io::{Read, Write};
+#[cfg(feature = "bonus:network")]
+use std::net::TcpListener;
 
 #[derive(Debug)]
 pub struct NetStack {
@@ -15,6 +19,7 @@ impl NetStack {
         }
     }
 
+    #[cfg(feature = "bonus:network")]
     pub fn bind(&mut self, port: u16) -> Result<()> {
         let listener = match TcpListener::bind(format!("0.0.0.0:{}", port)) {
             Ok(a) => a,
@@ -30,6 +35,7 @@ impl NetStack {
         }        
     }
 
+    #[cfg(feature = "bonus:network")]
     pub fn connect(&mut self, host: u32, port: u16) -> Result<()> {
         let h1 = (host & (0xFF << 24)) >> 24 as u8;
         let h2 = (host & (0xFF << 16)) >> 16 as u8;
@@ -44,6 +50,7 @@ impl NetStack {
         Err("Could not connect to address")
     }
 
+    #[cfg(feature = "bonus:network")]
     pub fn close(&mut self) -> Result<()> {
         if let None = self.stream {
             return Err("Cannot close a nonexistent socket")
@@ -53,6 +60,7 @@ impl NetStack {
         Ok(())
     }
 
+    #[cfg(feature = "bonus:network")]
     pub fn read_byte(&mut self) -> Result<i8> {
         return match self.stream {
             Some(ref mut stream) => {
@@ -65,6 +73,7 @@ impl NetStack {
         }
     }
 
+    #[cfg(feature = "bonus:network")]
     pub fn write_byte(&mut self, byte: u8) -> Result<()> {
         return match self.stream {
             Some(ref mut stream) => {