Added clover reporter

We use Atlassian's OnDemand Bamboo CI server, which uses the clover xml format to record simple statistics about code coverage.  We are already using the JUnit reporter, which works well with Bamboo out of the box.
I used [istanbul's](https://github.com/gotwarlost/istanbul/blob/master/lib/report/clover.js#L127) reporter output as a basis for this reporter. But further research into the xml format suggests that it doesn't matter what is in this field. For instance, the [PHPUnit reporter](https://github.com/sebastianbergmann/php-code-coverage/blob/d46cd5e9ac07e78aa060cd7975d5eca16419de38/src/CodeCoverage/Report/Clover.php#L70) for clover doesn't appear to even attach the `clover` attribute to the `coverage` tag. Also, the bamboo-gcov-plugin, which is a python utility for converting gcov coverage results to clover xml, at [line 90](https://bitbucket.org/atlassian/bamboo-gcov-plugin/src/1030f9c2982ec6826eceda059604817ed93f8bcf/gcov_to_clover.py?at=default#cl-90) they are inserting the name of the file `gcov_to_clover.py`.

I will run an actual build with this value removed and report back.