Add paddings to Stack and create Layout function

I tried to make an intuitive function to quickly create good-looking layouts. I liked the result, but I understand if you don't want to increase the API size right now, so this is only a suggestion. Anyway, the padding/margin code may be useful by itself.

This change is twofold:

- First, add unexported attributes `padding` and `marginBorder` to `Stack` that work as expected. I'm open to name suggestions here.

- Second, export a function `Layout` that creates a stack with automatic orientation, padding and border.

`Layout` works by giving only the topmost stacks a margin, and settings its orientation to "vertical". All nested stacks inside it are oriented perpendicular to its parent and have only padding, not margin. As a bonus, any `nil` value is converted to a stretchy space.

So for example, this code:

```go
w := ui.NewWindow("Test Window", 400, 250)
w.Open(ui.Layout(ui.NewLabel("Task Description"),
	ui.NewProgressBar(),
	nil,
	ui.Layout(ui.NewButton("Button 1"),
		ui.NewLineEdit("Edit"),
		nil,
		ui.Layout(ui.NewButton("Button 2"),
			ui.NewButton("Button 3"),
			ui.NewButton("Button 4")),
		ui.Layout(ui.NewButton("Button 5"),
			nil,
			ui.NewButton("Button 6")))))
```
Creates this layout:

![goui](https://cloud.githubusercontent.com/assets/231856/3353863/04dd1c24-fa8f-11e3-9bf6-6447619f4340.png)

I personally liked the results and will be using the function, merging or not. I'm also considering making a `Stretch` function, so you can write `ui.Layout(button1, ui.Stretch(button2), button3)` but right now I can't think of an elegant implementation because the stretching information is on the parent.