Add support for notifications API

[baransu]

I found that notifications is not covered in this library so I've added functions cover this part of API.

I've also tried to add tests to every new function but in seems that token used in tests has not access to notifications, or I'm doing something wrong. Also I've tried to test Notifications.Thread module but I have no idea how to test. It require thread_id but I have no idea how to find one valid. The one used in docs is some random and official documentation uses 1 as id which returns that I have no access to this repository.

I'm novice to elixir so if someone can review this or improve my additions it would be awesome.

[coveralls]

Coverage decreased (-1.7%) to 89.256% when pulling c9a6c53decaea4ff8ed5637e61e31cd1f10705f3 on Baransu:master into 4c2dd7e5506a2f295cb528d67ae16eb3de12a2f2 on edgurgel:master.

[coveralls]

Coverage decreased (-1.7%) to 89.256% when pulling c9a6c53decaea4ff8ed5637e61e31cd1f10705f3 on Baransu:master into 4c2dd7e5506a2f295cb528d67ae16eb3de12a2f2 on edgurgel:master.

[sdimkov]

Any progress on this?