Best Practice Validation of Timestamp Usage

Fixes #9.

XPath is run to ensure that all major ID-able elements have timestamps, and warns when elements do not have timestamps. In addition, elements with idrefs and timestamps are collated, and those idref/timestamp combos are used to ensure an element with that combination does exist. This check does not ensure that the referenced item is of a specific type though, as I wasn't sure that this is the right place for that (or that this is feasible to solve via generic XPath.)