Add Module Category for Content Staging

Problem/Motivation

Currently there isn't any category slightly related to content staging. I'm working heavily in the Content Staging initiative (https://groups.drupal.org/large-scale-drupal-lsd-projects-and-plans/cont...) and the list of modules keeps growing. It'll be nice if there was a category to filter these modules that support it and for new comers to be able to easily find the appropriate modules.

Proposed resolution

Add a module category for "Content Staging", I'm ok with "Content Staging", "Site Preview System", or anything related to it. I like content staging because then we can also put all the workflow, revision modules in that category.

List of modules that could be added to this category (only spent 60 seconds), the list is probably larger.

https://drupal.org/project/sps
https://drupal.org/project/sps_prototype
https://drupal.org/project/file_entity_revisions
https://drupal.org/project/state_machine
https://drupal.org/project/workbench_moderation
https://drupal.org/project/ers
https://drupal.org/project/collections
https://drupal.org/project/bean
https://drupal.org/project/nra
https://drupal.org/project/nra_ers
https://drupal.org/project/workflow
https://drupal.org/project/workflow_extensions
https://drupal.org/project/workflow_fields
https://drupal.org/project/workflow_named_transitions
https://drupal.org/project/og_workflow
https://drupal.org/project/iib
https://drupal.org/project/nra_workbench_moderation
https://drupal.org/project/revisioning
https://github.com/ericduran/drupal-revision_scheduler_sps (will be moved to drupal.org soon)