Problem:
System.IO.FileNotFoundException: Could not load file or assembly 'Microsoft.VisualStudio.SharePoint.WorkflowExtensions, Version=11.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified.
File name: 'Microsoft.VisualStudio.SharePoint.WorkflowExtensions, Version=11.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'

Causes/Solutions:
Our team faced this exception for several scenarios in Development environment. We fixed this problem in different combination for different scenarios.

Development Environment:
Windows server 2012
Visual Studio 2012
SharePoint Server 2013,
SharePoint Workflow 2013

Cause-1:
Workflow Manager Tool was not installed.

Solution-1:
Simply installed the workflow manager tools for SharePoint Workflow 2013.

Cause-2:
Workflow Manager Tool was installed but not updated with corresponding Cumulative update.

Solution-2:
As we know SharePoint workflow is managed by Workflow Manager 1.0 and its depends upon Service Bus 1.0. So need to update both version as well.
These two cumulative updates were working for us.

February 2013 Cumulative Update for Service Bus 1.0: KB2799752
February 2013 Cumulative Update for Workflow Manager 1.0: KB2799754

Cause-3:
Office Developer Tools (OfficeDevToolsforVS2012) was not installed on the DEV box.

Solution-3:
Installed the correct version of Office developer tools. We need to installed the OfficeDevToolsforVS2012 version.

Hopefully this will be helpful for other.