Adding post details in preparation for the API importer code.

This request adds the metadata @SamSaffron and I have been discussing.  I will need it for the import API endpoint I wish to add so that I can import the Google Groups messages from Parley.

I went with the name `PostDetail` which I realize isn't too awesome, but it avoids the pluralization problem with anything ending in data.  I tried to follow Sam's recommendations for everything else.

Just let me know if I need to change any of this.