more api functions :) - also closes #7 #8 #9

ADD API Calls with tests:
- GET /user
- POST /clear-completed
- PUT /task/{id}
- DELETE /task/{id}

FIXES:
- Task.Challenges is a Challange not a List<Challenge> (see habitrpg/src/model/task.js)

Changes:
- DateTime Asserts in Test, sometimes the calls had a millisecond differences, and then the tests failed because of that, I had to replace these tests
- GetTasksAsync now returns a habit/todo/daily/reward class instead of "task" ( see TaskConverter)
- History.Date need to be unix timestamps and not full readable Datetime like the other DateTime Properties, so I needed an other Converter just for this
- HabitRPGClient now also uses /api/v2/ as BaseAddress so that all calls don't have to add them

I hope I didn't forgot something.