Define two annotation layers

Adds a function to create an annotation layer for intraline differences
in gr-diff-builder (as a vanilla object) and introduces an annotation
layer for ranged comment highlights (as a Polymer element).

The interface for annotation layers may become more formalized later,
but at this stage, an annotation layer needs to be an object with the
following two methods.

* `annotate : Function<HTMLElement, GrDiffLine, GrAnnotation>`
* `addListener : Function<Function<Number, Number, String>>`

The `annotate` method applies that layer's annotations to the provided
DIV.contentText element and line object. The annotation library is
provided for convenience. The `addListener` method registers a listener
for when an annotation layer says that a range of lines needs to be
updated.

As of this change, the builder is not yet making use of these new
layers, leaving functionality as-is.

Change-Id: I1083aaeb7e1d6eeff46687fa5cf7b52bc6bb834d