Eliminate some warnings

examples/hello_world.rs

@@ -10,7 +10,7 @@ use cocoa::appkit::{NSApp, NSRect, NSPoint, NSSize,
 
 fn main() {
 	unsafe {
-		let pool = NSAutoreleasePool::new(nil);
+		let _pool = NSAutoreleasePool::new(nil);
 
 		let app = NSApp();
 		app.setActivationPolicy_(NSApplicationActivationPolicyRegular);

src/appkit.rs

@@ -9,7 +9,7 @@
 
 #![allow(non_upper_case_globals)]
 
-use base::{ObjCMethodCall, id, SEL, nil, NSInteger, NSUInteger};
+use base::{ObjCMethodCall, id, SEL, NSInteger, NSUInteger};
 use libc;
 
 #[cfg(target_word_size = "32")]

src/base.rs

@@ -32,7 +32,8 @@ pub type NSInteger = libc::c_long;
 #[cfg(target_word_size = "64")]
 pub type NSUInteger = libc::c_ulong;
 
-pub static nil: id = 0 as id;
+#[allow(non_upper_case_globals)]
+pub const nil: id = 0;
 
 extern {
     pub fn class_addMethod(cls: Class, name: SEL, imp: IMP, types: *const libc::c_char) -> bool;