XUnit Reporter Writes to stdout, falls back to console.log

Fixes mochajs#1674; provides support *much* needed for [mocha-phantomjs](https://github.com/nathanboktae/mocha-phantomjs) (see [mochajs#133](nathanboktae/mocha-phantomjs#133), [mochajs#220](nathanboktae/mocha-phantomjs#220), etc).  Maintains support for running the XUnit reporter in a browser (unlike the previously reverted PR mochajs#1068).

ping @alemangui, @nathanboktae

Thanks all.

lib/reporters/xunit.js

@@ -109,6 +109,8 @@ XUnit.prototype.done = function(failures, fn) {
 XUnit.prototype.write = function(line) {
   if (this.fileStream) {
     this.fileStream.write(line + '\n');
+  } else if (typeof process === 'object' && typeof process.stdout === 'object') {
+    process.stdout.write(line + '\n');
   } else {
     console.log(line);
   }