Add instructions on using Drush Rebuild for local environment rebuilding

I added instructions on how to use [Drush Rebuild](https://drupal.org/project/rebuild) to quickly and easily rebuild a local Warmshowers development site. (Disclaimer: I wrote the Drush Rebuild extension.) 

Drush Rebuild uses the settings contained in a site-specific YAML config file to import the database from Dropbox, enable modules, set variables, define permissions, and log the user into the site. Eventually the scripts from assets/devscripts could be incorporated too. The idea is to have one command (`drush rebuild @warmshowers.local`) that is repeatable and reliable in rebuilding the local dev site.

I also included an example Drush alias and adjusted the documentation so that those who want to use this approach, can.