Move API to instance

[wybiral]

The new interface should look something like this:

• h := hookah.New() creates a new instance of hookah
• r, err := h.NewInput("proto://args") create a new input (ReadCloser)
• w, err := h.NewOutput("proto://args") create a new output (WriteCloser)
• h.RegisterInput("proto", handler) register input handler
• h.RegisterOutput("proto", handler) register output handler

This will allow the current default handlers to all be registered the same way as user-defined ones and will make it easier to enumerate them (so they can be listed by the application).

[wybiral]

#14