Improve the marking of sent events

In generate_output(), we were marking events as sent by changing the XML
tree and saving it for each form event sent. This change replaces it
with a "sent event index" which simply writes out the form event to a
file and loads it back in if we use the --resume switch.

I've seen a 12% speed improvement during testing.
Merge branch 'develop' into sent-event-index
Update warning about unsent events

Updated the warning message to check the log file first. Also, after
discussing it with some of the other developers, I decided to make the
sent_events file more human-readable. In doing so, I implemented a
faster approach to persisting the list of sent events.  Instead of
pickling the list each time, the latest event simply has to be appended
to the file.
Delete sent_events.idx before a new run
fixing merge conflicts in tests
Merge branch 'ufl-taeber-sent-event-index' into develop