Include the whole document in dom.serializedSize, not just the body

XMLSerializer::serializeToString gets us what we want. For some reason
it's not exactly equivalent to the actual file size, but it's within 10
bytes.
Merge pull request #3 from aroben/xmlserializer

Include the whole document in dom.serializedSize, not just the body
dom-prof 0.3.1
Fix example syntax