Expands the `<content>` element to remember logical DOM

It's now capable of rerendering if logical DOM changes.

Notes:

* lightChildren, if present, indicates the logical child views. lightParent will be set for those elements.
* lightChildren is an array, because that seemed like the "simplest thing that could possibly work". We could expose linked list APIs instead (firstChild, nextSibling, etc) if we wanted.
* addLightChild/removeLightChild are a convenience, but not recommended for adding large number of items because they automatically call distributeContent(). Another option would be to schedule it and do it lazily.
Merge pull request #1017 from Polymer/content_lightchildren

Expands the `<content>` element to remember logical DOM