.NET Standard support

[MFinkBK]

I know that this pull request most likely isn't accepted or even considered, since this project is out of maintenance. I still post this so that any interested user can have a look at it. Who knows, maybe someone wants to revive the project from the dead.

This pull request does the following:

• remove Windows Phone, WinRT and iOS Classic projects - they are not supported by latest Visual Studio 2017
• change the project files so that the SQLite.Net and SQLite.Net.Async projects are compiled as .NET Standard projects, not as PCL projects anymore.
• cleans up the projects
• changes the version number to 4.0.0-beta1, so that NuGet packages would have different names

This PR would benefit all that have to use the SQLite.Net-PCL NuGet package, e.g. when using the commercial SQLcipher project that is using the ISQLitePlatform interface.

[CarLoOSX]

Hello!! I would like to get your beta. What can I do ? Could I get the last commit ?? Thank you @MFinkBK

[MFinkBK]

You can clone the repository at https://github.com/MFinkBK/SQLite.Net-PCL, open the SQLite.Net.sln and compile the assemblies for yourself.

Note that as @oysteinkrog doesn't work on this project anymore, the changes and this pull request are purely for documentation purposes. The way to go would be to use @praeclarum's version of SQLite.Net, but unfortunately it lacks the ISQLiteApi interface that is needed to use SQLcipher libraries together with SQLite.Net. Someone should port this to @praeclarum's version some time...