use console.debug() in browser when available (closes #600)

also removes a branch for each logging call, slightly improving
performance in the browser.

src/browser.js

@@ -4,7 +4,6 @@
  * This is the web browser implementation of `debug()`.
  */
 
-exports.log = log;
 exports.formatArgs = formatArgs;
 exports.save = save;
 exports.load = load;
@@ -170,18 +169,14 @@ function formatArgs(args) {
 }
 
 /**
- * Invokes `console.log()` when available.
- * No-op when `console.log` is not a "function".
+ * Invokes `console.debug()` when available.
+ * No-op when `console.debug` is not a "function".
+ * If `console.debug` is not available, falls back
+ * to `console.log`.
  *
  * @api public
  */
-function log(...args) {
-	// This hackery is required for IE8/9, where
-	// the `console.log` function doesn't have 'apply'
-	return typeof console === 'object' &&
-		console.log &&
-		console.log(...args);
-}
+exports.log = console.debug || console.log || (() => {});
 
 /**
  * Save `namespaces`.