Fixes #158. Major update to code organization. Base functions to
calculate retention are now in utils.py. Code to integrate with Anki is
broken up into two files: stats_true_retention_extended.py and
stats_true_retention_extended_old.py. Code to write to the new stats
page introduced in 2.1.28 is in its own JS file setStats.js. init.py
has been updated to facilitate backwards compatibility with older
versions of Anki. Also incorporates PR by @zjosua to not use deprecated
fmtTimeSpan
Fixes #158. Major update to code organization. Base functions to calculate retention are now in utils.py. Code to integrate with Anki is broken up into two files: stats_true_retention_extended.py and stats_true_retention_extended_old.py. Code to write to the new stats page introduced in 2.1.28 is in its own JS file setStats.js. init.py has been updated to facilitate backwards compatibility with older versions of Anki. Also incorporates PR by @zjosua to not use deprecated fmtTimeSpan