Rudimentary track-changes support

This patch-set introduces rudimentary support of track-changes. At the moment, it does what I imagine would be the sensible default for most people: read in insertions, and silently ignore deletions. (Previously, both were ignored).

The framework is in place, though, for easily extending this through a more flexible set of options. Once this is in place, what would you think about `--track-changes=accept | reject | all`?

  - `accept` would be the default, and would put in the insertions, and silently ignore the deletions.
  
  - `reject` would do the opposite, keep the deletions, and silently ignore the insertions.
  
  - `all` would leave everything in, for the purposes of scripting. Putting aside all the critic-markup questions of adding change types to the pandoc ast, we could easily span. We keep the change date and the author info, so we could have something like:

~~~
This is an 
<span class="insertion" author="Jesse Rosenthal" date="2014-06-25T08:31:00Z">insertion</span>, 
and this is a <span class="deletion" ...> deletion</span>
~~~

People could then script and accept and reject by time or author. (Note that the time is currently kept in ooxml's T-delimited ISO 8601 format. It can easily be converted to something else during the parsing if that's preferable.