Simplify example by using AlexaSkillsKit

[choefele]

As an exercise, I rewrote the example to use AlexaSkillsKit to simplify parsing and generating Alexa messages. The functionality is still the same, but I think the sample is easier to read and understand. Let me know if this is useful

I tested the sample with swift build && cat ../../AlexaSkillsKit/Tests/AlexaSkillsKitTests/launch_request.json | .build/debug/swiftcommand

• echo function enabled: returns whatever was passed in to swiftcommand
• greetResponse enabled: returns standard response with "Hello from Swift"