Replace Rhino Mocks with Moq

[gsomix]

https://github.com/moq/moq4

Possibly solution for #7.

[ForNeVeR]

Needs further investigation. According to their issue tracker, there're still no .NET Core support. Although, the corresponding pull request was merged some time ago.

See also: Moq on .NET Core. Seems to be possible.

[ForNeVeR]

Okay, it seems that they have .NETStandard 1.3 support starting from 4.6.25-alpha. Depending on the alpha builds is not the best course of action IMO, but that's the only option I can see insofar.