convert path separator to slash on windows.

windows path separator is backslash, so test is failed. #1


Also, when I try to generate report with covertura reporter as below, the test/js/helper.js will cause "Fatal error: Object.keys called on non-object" when being traversed.
That I want to have control to generate different types of reports.
<pre>
exports.process = function (grunt, task, context) {
	var instrumenter = new istanbul.Instrumenter();
	var reporter = istanbul.Report.create('cobertura', {
		dir: context.options.report
	});
</pre>