Add option to run tests in parallel

For test that are unrelated, it would be better to let them run in parallel than wait for them to run in series.  This adds an option to run the tests in parallel with an option setting like:

```javascript
options: {
  async: {
    parallel: true
  }
}
```

The async option is deleted before the options object is passed to `casperjs()`, since I believe those are all supposed to be Casper/Phantom options.