For test_failures.py:
* Replaces HistoryReport with a TestHistory class that has
get_history_by_revision() and get_history_by_date() methods. They
both return a list of ReportEntry tuples that can be used to
construct a Report instance.
* Adds Python unit test cases for the Report and ReportEntry classes.
* Creates Wildcard class as separate concept from Missing class.
* Enables --sinceDate and --untilDate with a warning that the script
may not return a complete result set.
* Adds support for running the script with Python 3.
For update_test_lifecycle.py:
* Introduces Config namedtuple to represent the test lifecycle model.
* Adds Python unit tests cases for the update_tags() function.
* Takes advantage of the partial grouping so that computing summaries
for (test, task, variant), (test, task), and (test,) combinations do
not require re-processing the entire result set.
