auto merge of #5278 : brson/rust/logplusplus, r=brson

r?

`log` can  polymorphically log anything, but debug!, etc. requires a format string. With this patch you can equivalently write `debug!(foo)` or `debug!("%?", foo)`.

I'm doing this because I was trying to remove `log` (replacing it with nothing, at least temporarily), but there are a number of logging statements that just want to print an arbitrary value and don't care about the format string.

I'm not entirely convinced this is a good change, since it overloads the implementation of these macros and makes their usage slightly more nuanced.

src/test/run-pass/log-poly.rs

@@ -0,0 +1,10 @@
+enum Numbers {
+    Three
+}
+
+fn main() {
+    debug!(1);
+    info!(2.0);
+    warn!(Three);
+    error!(~[4]);
+}