[Breaking] Added .NET Standard support

[niemyjski]

This commit added support for .NET Standard, the future of .NET...

• This library works cross platform and is built with .Net Standard 2.0 (supports .NET core as well as .NET Framework 4.6.1)
• Version is 0.9 and will be a prerelease tag. As things could still be changed or features like configuration brought back before release.
• Moved the build system to appveyor
• No more dependencies on System.Web, which means the samples have also been removed.
• We have removed System.Configuration as it's legacy and support is limited to windows only. Configuration should be done in code now which is much less error prone... PR would be welcome to add support for the new configuration system.
• RestSharp has been internalized and the licenses and noticed have been included. We only internalized for the time being because RestSharp hasn't been updated in a long while and there is no official .net standard support.... Once the new client supports netstandard we will remove this internalized version.

[niemyjski]

I'm merging this as build's currently can't be triggered on branches until after this pr (I can't see any other way to do it with appveyor). No artifacts will be deployed without a pre tag, any commit not tagged will have a -pre tag and we can always revert the single commit if needed.