Removed obsolete generic overload of BindableProperty.Create(). Small C# 6.0 refactorings

[zuckerthoben]

Hi, this is my first pull request ever!

While using your control I saw messages from Visual Studio stating that BindablePropety.Create<TDeclarer, TPropertyType> is obsolete and will be removed. Reasons for this change are stated here: https://forums.xamarin.com/discussion/63268/bindableproperty-pieces-obsoleting

I then tried to replace the obsolete implementation with a safe one. Ran it in my app and seems to work as before.

While I was at it I could not resist to execute some automatic code refactorings from VS 2017.

Thats it. I hope I did this reasonably right.

Greetings