[api] Write XML coverage report to coverage.xml

Ref #161.

Author: mmalecki

lib/vows/coverage/report-xml.js

@@ -1,4 +1,4 @@
-var util = require('util'),
+var fs = require('fs'),
     file = require('./file');
 
 this.name = 'coverage-report-xml';
@@ -77,5 +77,5 @@ this.report = function (coverageMap) {
         + all.xml
         + '</all>\n</data>\n</report>\n';
 
-    util.print(all.xml);
-};
+    fs.writeFileSync('coverage.xml', all.xml);
+};