Pre-release of .NET Core full support

#94 has been completed, and this is the result: a pre release of the latest version of RawRabbit that targets netstandard1.5 and net451.

Make sure that https://ci.appveyor.com/nuget/rabbitmq-dotnet-client-ci is added to your nuget feeds in order to be able to grab the pre release of RabbitMQ.Client