Refactor docker.toml configuration into sections

[kofalt]

Right now, we express three fundamentally different concepts:

• Image name, parent image name
• Image settings (which you might want to extend in a child folder)
• Run targets (default, build, etc)

The current format has the following problems:

• The per-target setting inheritance is confusing and overly clever.
  • Worse, it conflates the default default run target with the default settings.
• Settings are loaded in an unintuitive manner; right now, in Boxen, most images can omit the build target's command... unless they change the default target's command. What?
• Placing the image's name in the default run target and the upstream/index image name in the build target is another strange facet.
• Surprising behaviour possible: specify different images in different targets; hilarity ensues.

... The list goes on. Basically, not a bad first draft, but let's scrap it and move on.

Current format:

#Settings inheritance is a complete mess

# Why is "default" holding settings and commands?
[default]
    command = [ "/bin/echo", "Hello from docket!" ]
    dns = [ "8.8.8.8", "8.8.4.4" ]
    image = "docket.polydawn.net/ubuntu/12.04"

[bash]
    command = [ "/bin/bash" ]

[build]
    command = [ "/cwd/build.sh" ]
    # Why is the image name mentioned in a target?
    upstream = "index.docker.io/ubuntu/12.04" 
    index = "ubuntu:12.04"

Proposed new format: only settings inherit.

# Hey, this section makes intuitive sense!
[image]
    name     = "docket.polydawn.net/ubuntu/12.04"
    upstream = "index.docker.io/ubuntu/12.04"
    index    = "ubuntu:12.04"

# These apply to all targets, and all images in child folders.
[settings]
        dns = [ "8.8.8.8", "8.8.4.4" ]

# Target settings do not apply to any other file - only this target, for this image.
[target.run] # Renamed default target to run, see #30
    command = [ "/bin/echo", "Hello from docket!" ]
    ports = [ "80", "80" ]

[target.bash]
    command = [ "/bin/bash" ]

[target.build]
    command = [ "/cwd/build.sh" ]
    # This ports setting only applies to the build target!
    ports = [ "80", "8080" ]

Advantages: correct all the problems enumerated above, vastly simplified.
Disadvantages: lose some niche features like only specifying the bash target once.