Different people will want to calculate the points for their burndown charts in different ways. This commit breaks a lot of cross module dependencies to add support for a custom `PointsCalculator` interface, which follows the Strategy design pattern for calculating the points over time to show on the burndown chart. Four calculators are made available by default (see `README.md`). Additional calculators may be added over time or by PR. Closes #23 by providing the `taiga` calculator.