Add AnchorPadding with percents to Layer class

It is possible to set AnchorPadding in pixels or percents relative to the
Composition's width/height.
AnchorPaddingType in Layer specifies unit type of AnchorPadding.
LayerBuilder has overloaded AnchorPadding function that accepts Unit
structure and sets both AnchorPadding and AnchorPaddingType.
AnchorPadding uses pixels by default, API is backwards compatible.

Example of usage: write watermark at the bottom of the image with 30%
"margin":
LayerBuilder.Text.Text("(c) Copyright 2014")
  .Anchor(AnchorStyles.BottomCenter)
  .AnchorPadding(Unit.Percentage(30));