Add nginx support.

This commit adds nginx support. You can now decide between apache2 and nginx by setting the correct value in default['drupal']['webserver'].

Update the link to Opscode cookbook repository in the README.md
Disable modules

Add the possibility to disable modules.

This commit change the attribute 'drupal[:modules]' into
'default['drupal']['modules']['enable']' to install and enable the module and adds the attribute 'drupal[:modules][:disable]' to disable modules.