Support margin and padding

[da1nerd]

This could be implemented in several ways, but I think the simplest and most flexible way would be to create a dedicated component that you could wrap other components with.

for example, the following code could add a 10 pixel margin around the children. The margin component would automatically add constraints to it's children to create the proper margin.

component = Margin.new(10)
component.children = [
  # add children
]