Add new script to predict ping response time

classify_ping_response.py loads the pandas DataFrame
produced by clean_ping_data.py and fits a classifier to
selected features, with the goal of predicting whether a ping
will get a response within a specified time limit.

The current version uses a random forest classifier with
10 estimators and sets the time limit for a successful ping response
to 30 seconds. Included features are availability, client,
time_sent_success_local, sec_since_online, sec_since_pageload,
and timezone.

Using all of the features and 3-fold cross-validation,
the precision of predicted fast ping responses (how often a
predicted fast response is actually under 30 sec) is about 30%,
and the recall (the fraction of actual fast responses that the
classifier identifies) is about 10%. The most important features,
with approximately equal weights, are time_sent_success_local,
sec_since_online, and sec_since_pageload; the remaining features
are substantially less important.