expose serializer method for child nodes

[ronco]

Fastboot needs to serialize the contents of the body & head elements, but not those tags themselves. Currently those tags are included in the output generated by serialize leading to nested body & head tags (as seen here). This PR exposes a serializeChildren method to enable this behavior. I'm glad to update/take a different approach if a different solution is preferred.

[rwjblue]

It seems that we are basically making serializeChildren similar to elem.innerHTML and serialize is like elem.outerHTML.

This looks good to me.

@tomdale / @krisselden - Any issues with this?

[tomdale]

Seems fine to me.

[chancancode]

Nice! :D