Upgrade the site to .NET 4.5.1.

Upgrade to the latest version of .NET for all the new platform features. This also updates NuGet Package Restore to the latest version of NuGet (v2.8) which has better support for .NET 4.5.1 and VS2013. 

See http://go.microsoft.com/fwlink/?LinkId=235367 for details of the changes made to the web.config file.