Migrate Client side to Netstandard

[Shaddix]

depend on #14 (SignalR should be migrated to .net core first)

[Shaddix]

Test application could not be migrated yet due to severe issue in Realm itself (realm/realm-dotnet#1663).
Here's the workaround: https://forums.realm.io/t/ireflectabletype-is-defined-in-an-assembly-that-is-not-referenced/792/6

[Shaddix]

The workaround works for System.Reflection, but not for System.Net.Http, so we're still stuck. Waiting for the fix