fixing junit report generation when folder doesn't exist.

On windows when specifying a output folder the folder structure of the tests must manually be created. This change automatically creates the folder structure as needed.
```
fs.js:427
  return binding.open(pathModule._makeLong(path), stringToFlags(flags), mode);
                 ^
Error: ENOENT, no such file or directory 'C:\development\ui-tests\tests\Security\Login.xml'
    at Object.fs.openSync (fs.js:427:18)
    at Object.fs.writeFileSync (fs.js:966:15)
    at C:\development\code\Tests\node_modules\nightwatch\runner\reporters\junit.js:36:10
    at fs.js:266:14
    at Object.oncomplete (fs.js:107:15)
```