Improvement/legend

[boygirl]

This PR includes several improvements for VictoryLegend:

This may be a breaking change for VictoryLegend

• Adds a border component that can be styled or replaced with a custom component
  new props: borderComponent={<Border/>} and borderPadding
  style / event namespace: "border"

• Adds new primitive component Border
  renders a rect element
  props: common props + width, height, x and y

• Adds a title component with supporting props for positioning and center titles
  new props: title, titleComponent={<VictoryLabel/>}, titleOrientation, centerTitle
  style / event namespace: "title"

fixes FormidableLabs/victory#714
fixes FormidableLabs/victory#729
fixes FormidableLabs/victory#736