Feature: Allow testem to run specific test module

Adds `--module` command line option to `ember test` and `ember test
--server`. The `--module` option tells QUnit to run the specified test module.

Example usage `ember test --module "Module Name Here"`
Perhaps, something like:

```javascript
var params = [];

if (options.module) {
  params.push('module=' + options.module);
}

if (options.filter) {
  params.push('filter=' + options.filter);
}

originalContents['test_page'] = originalContents['test_page'] + '?' + params.join('&');
```