CORE: workbench permissions extension module, & README updates

## Ready State: Yes

This pull request is related to the following JIRA tickets:  https://imagex.atlassian.net/browse/LDC-219

## Requirements & Dependencies

This module requires
* Features
* Rules
* Workbench, Workbench_moderation, workbench_access, workbench_media
* imagex_user_roles

## Deployment Instructions

* Merge code.

## Usage/Test Instructions

* Ensure that workbench moderation defaults are correctly enabled on install of this component
* Confirm that imagex_workflow_permissions correctly applies workflor permissions for imagex_user_roles
* Visually view the changes on the homepage.