Uwp support

[anton-yashin]

Hi! Changes due #14 latest UWP SDK's doesn't work properly with netstandard 1.0:

• LightMock target framework updated to netstandard2.0
• LightMock.Tests target framework updated to netcoreapp3.1
• tests migrated to xunit
• added test runners for android, ios and uwp platforms.

If there too many changes i can just update frameworks.

Thanks.

[anton-yashin]

Hmm. No vs2017rc at appveyor. May be i need to make changes in appveyor.yml?

[seesharper]

I guess it is about time to update the build for this project :) I'll take a look at it

[seesharper]

Hi @anton-yashin and thanks a lot for your work migrating the tests to xUnit.
I have however decided to just bump to netstandard2.0 and leaving out the tests that specifically targets the various platforms.
netstandard2.0 is supported by all these platforms so there should not be any need to test this.
But again, thanks a lot for your work 🎂
The new package is available on NuGet as version 2.0.0

[anton-yashin]

Thanks :-)