Port to Botframework v4

[tonysurma]

(originally created by @toolboc)

Microsoft has recently announced an upgrade of the currently used Bot Framework v3 to a new and improved Bot Framework v4. While still in an experimental phase, this update it a full change to the previous version of the framework, as such, it will likely involve creation of a brand new project which takes advantage of these new features. Given the relative compactness of the current codebase, it would make sense to begin this port as soon as possible to accommodate for the future roadmap of the Bot Framework SDK.

See: https://github.com/Microsoft/botbuilder-dotnet