Use attributes for native module ABI and link name

This patch changes how to specify ABI and link name of a native module.

Before:
  native "cdecl" mod llvm = "rustllvm" {...}

After:
  #[abi = "cdecl"]
  #[link_name = "rustllvm"]
  native mod llvm {...}

The old optional syntax for ABI and link name is no longer supported.

Fixes issue #547

src/lib/rand.rs

@@ -3,7 +3,8 @@ Module: rand
 
 Random number generation
 */
-native "cdecl" mod rustrt {
+#[abi = "cdecl"]
+native mod rustrt {
     type rctx;
     fn rand_new() -> rctx;
     fn rand_next(c: rctx) -> u32;