Update for week 1 of Milestone 3

Last week I managed to do the following tasks for #SAOC2020:
- refactorized the code (added APIRequest struct which contains the request body, url and headers for the request)
- tested the library
- added (almost all) unittests

The plan for the next week:
- comment the code
- add the rest of the unittests
- start gcalendar library implementation

Milestone 1 updates: [0]
Milestone 2 week 1: [1]
Milestone 2 week 2: [2]
Milestone 2 week 3: [3]
Milestone 2 week 4: [4]
Milestone 3 week 1: [5]

[0]: https://forum.dlang.org/post/eaefvlbgikujonfjjzqi@forum.dlang.org
[1]: https://forum.dlang.org/post/jezqwzpeknpxhmqgkewe@forum.dlang.org
[2]: https://forum.dlang.org/post/kffvsmmxrkftxlbqklsd@forum.dlang.org
[3]: https://forum.dlang.org/post/yvbnsycuxqjfxghqajbu@forum.dlang.org
[4]: https://forum.dlang.org/post/jwasbvtjbofftybnsqro@forum.dlang.org
[5]: https://forum.dlang.org/post/gimihhnxzbkeodvrotoy@forum.dlang.org