Add testdir config to handle matching classname

Hi,

this pullrequest aims to allow generate xunit report for Sonar with classname matching test file.
It uses a new property `testdir` in package.json config/mocha-sonar-reporter

Example:
```xml
<testsuite name="Mocha Tests" tests="2" failures="0" errors="0" skipped="0" timestamp="Mon, 08 Dec 2014 10:00:39 GMT" time="0.002">
   <testcase classname="bar-spec" name="Bar #salut should return salut $name" time="0.001"/>
   <testcase classname="toto/foo-spec" name="Foo #hello should return hello $name" time="0"/>
</testsuite>
```