Also report lint-free files

Hi @derekprior

Here is a little change for you to review! And merge in if you feel it.

Not sure whether or not it's tied to my Jenkins / Junit plugin setup, but I had to make sure to also report files that are marked as lint-free to be able to report back the results. And properly exit the build with a success status.

Example running `jshint jshint-junit-xml-formatter.js --reporter jshint-junit-xml-formatter.js`:

```xml
<?xml version="1.0" encoding="utf-8"?>
<testsuite name="jshint" tests="0" failures="0" errors="0" >
	<testcase name="jshint-junit-xml-formatter.js"></testcase>
</testsuite>
```

Thanks for your reporter, I was really glad to find one while setting up this jshint job.