Add the Application Pool Identity account type for IIS7+ application poo...

Since the recommended [identity type](http://www.iis.net/learn/manage/configuring-security/application-pool-identities) to use for IIS7 + is AppPoolIdentity i think this should be an available option when creation new application pools using the Windows extention.
This change allows this.

Note, although the included Microsoft.Web.Administration.dll lib is version 6.1 it still supports this identity type. Regardless i would recommend upgrading to the latest [version](https://www.nuget.org/packages/Microsoft.Web.Administration) And change the reference to use nuget instead of including the library with the source. (since the BM SDK is also using nuget it makes sense).