v2.0.0: Added 2020-2021 Constants, Updated Social Security Tax Logic (#19)

* Added 2020-2021 constants
* Updated Social Security tax logic
* Updated unit tests to use latest values
* Updated example web application to use 2020-2021 constants

v1.0.2

Added 19-20 constants (#12)

v1.0.1

Fixed bug where student was being added as spouse (#2)

v1.0.0

Fixed a bug where parentTotalIncome was never getting set