Fix VSSDK links to MSDN in documentation

This change uses the **Additional Reference Links Plug-In** for SHFB to enable the documentation produced for the VSBase library to properly link to Visual Studio SDK topics at MSDN.

1. A new project **AdditionalSdkDocumentation.shfbproj** was created.
  * This project includes the [Visual Studio SDK assemblies as documentation sources](https://github.com/tunnelvisionlabs/vsbase/blob/5cea20223dbef0addf712bbf4829d060ff396c3b/AdditionalSdkDocumentation/AdditionalSdkDocumentation.shfbproj#L30-L42).
  * When the project was added to the solution, the Visual Studio configuration manager tool was used to disable building this project for all solution configurations.
2. The Additional Reference Links Plug-In was configured for the VSBaseDocumentation project. The **AdditionalSdkDocumentation.shfbproj** project was added as an Additional Reference Project, and the **WebsiteSdkLinkType** property for the project was set to **Msdn**.

   ![image](https://f.cloud.github.com/assets/1408396/2316254/758dab3e-a330-11e3-9d03-ea0aa4753156.png)

This update results in all links except for one reference to `ProvideCodeBaseAttribute` getting resolved. So far, this link remains unresolved since SHFB does not support the NuGet package manager and no other project in the solution currently references the **VSSDK.Shell.11** or **VSSDK.Shell.12** NuGet packages.