Add "rebuild" command (and deprecate "cc all") for Drupal 8

Drupal 8 has recently added a way to ["rebuild" a Drupal 8 site](https://drupal.org/node/2153725) after changes such as rebasing get the CMI files out of sync with the database (at least, I *think* that's what goes wrong; as a D8 n00b I'm not entirely sure). Unfortunately, the recommended core method is a bit of a pain. I bet Drush can do it better.

As per Moshe's suggestions in #drupal, since rebuilding won't work unless the site is bootstraped to a lover level than the `cache-clear` command, and since `drush_rebuild()` calls `drupal_flush_all_caches()` anyway, I've made `cache-clear all` deprecated; calling it under Drupal 8 will prompt the user to use the "rebuild" command instead. This may not be desirable after all, however, since I'm noticing that a rebuild takes a noticeably longer time to execute than just a cache clear.