Add SkypeCommand object to chat message handler
Move chat commands to objects

The previous implementation of closures for chat message commands had
the disadvantages of being harder to test, making the engine file large,
    not being pluggable, and not allowing more that one handler to
    execute for a single skype command. This commit moves the closures
    into tested command handler objects, and allows the chat message
    handler to call all of it's handlers and execute as many as want to.