A2ensite debian systems

Addresses issue https://drupal.org/node/2259119

The issue is that Rush isn't taking into account that on Debian systems the vhost file needs to 'enabled.' This usually happens via the a2ensite command. I've added a function to attempt to run this command on Linux machines.

To test:
* Checkout this branch of rush
* Run drush rush tests.host
* Check the vhosts files loaded by apache
-- (on linux): sudo apache2 -S
-- (on OSX): sudo apachectl -S

Be sure to test on Debian systems, as well as OSX.