Add aggregation script and clean up processing script

A number of commits to clean up the processing script, fixing a number of small issues and adding features necessary for the aggregation script. The aggregation script allows a central server to pull data from a number of school servers (each running olpc-journal-processor), and then merges the results to produce a couple of CSV files, ready for statistical analysis.

The individual commit messages should adequately describe all the changes made. Each commit is pretty much self-contained.