Release 3.0.0

[nigel-sampson]

I've put this here to consolidate some of the work being done for the 3.0.0 release.

At a broad stroke I'm trying to include the following.

• Bug fixes and minor changes that would result in breaking changes such as CalburnApplication does not call AssemblySourceCache.Install() #147, Refactor BindingScope #152, Does UriFor<T> make sense any more? #176
• Xamarin Xamarin - Native & Forms #142
• Windows 10 / Universal Windows Platform Windows 10 Universal App Platform #141
• WinRT State helpers Dealing with Navigation State in WinRT #95
• Removal the extensions projects. Do the Extensions assemblies still make sense? #191
• Shift to Shared projects. Windows 10 Universal App Platform #141
• Remove Windows 8 support. Supported Visual Studio versions and Platforms #209
• Have NET40 and SL5 use Caliburn.Micro.Platform.Core for consistency.
• Then move to it's own separate folder.
• Tidy up CachingFrameAdapter as a subclass of FrameAdapter.