Added handling of 'targetFramework' attribute to HttpRuntime

The 'targetFramework' attribute of the `<httpRuntime>` section in Web.config was added in .NET 4.5 (see blog post in [1]) and is automatically set to "4.5" by newer Visual Studio web templates. Up until now, this resulted in a crash when running on Mono, because the element was not recognized when the config file was deserialized.

This PR fixes the crash by introducing the element in HttpRuntime and HttpRuntimeSection. Note that this does not implement any of the "quirks mode" / fallback behavior mentioned in the blog post and is currently not used in any such way inside Mono.

Changes released under MIT/X11.

[1] http://blogs.msdn.com/b/webdev/archive/2012/11/19/all-about-httpruntime-targetframework.aspx