Add support to locate the Drupal root

This makes it so that the Drupal installer detects where Drupal is installed, and then will install it to `sites/all/modules` or `modules` depending on what Drupal version they're running.

Fixes #44.

Ripped most of the code from [DrupalInstaller](https://github.com/RobLoach/composer-installer/blob/master/src/Drupal/ComposerInstaller/DrupalInstaller.php#L36), with some touch up and some new additions from the latest [Drush](http://drupal.org/project/drush).

Thanks!