README: Virtualenv installation method added
README: text wrapped to 80 columns
License changed from GPLv3 to Apache v2
Source code wrapped to 80 columns
Trailing white space removed
Strengthener method made generally available via settings
Proportional algorithm handled in calc_future

Also, the functions that calculate strengthening and weakening have been
extracted into global functions so that they can be used without actual
WordPair and WDict objects.
Practice page: PgUp/PgDown control made generally available

The feature was implemented in the followings commits:

- 3ec02e9cb7097b0cc6f15e51d20aff36a0123f48
- d794d2b121e5c7e1f6e7cf1491719c278ff8b7a8
Early practice made generally available

Early practice was implemented in the following commit:
1965e85beeb610a3a55cabe3e721515464eb4c1b
Unnecessary line removed
Framework for hidden extras added
Practice page: showing date/strength/dimness information made into an extra

Available by adding the 'p' flag to the list of extras.
TODO updated