Add direct messages API calls

[rudineirk]

API Calls documentation:
https://rocket.chat/docs/developer-guides/rest-api/im/create/
https://rocket.chat/docs/developer-guides/rest-api/im/open/
https://rocket.chat/docs/developer-guides/rest-api/im/close/
https://rocket.chat/docs/developer-guides/rest-api/im/list/
https://rocket.chat/docs/developer-guides/rest-api/im/history/

[codecov]

Codecov Report

Merging #41 into master will increase coverage by 0.26%.
The diff coverage is 86.91%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
+ Coverage   85.84%   86.11%   +0.26%     
==========================================
  Files          19       24       +5     
  Lines         318      425     +107     
==========================================
+ Hits          273      366      +93     
- Misses         45       59      +14

Impacted Files	Coverage Δ
rocketchat/calls/im/open_room.py	75% <75%> (ø)
rocketchat/calls/im/close_room.py	75% <75%> (ø)
rocketchat/api.py	89.04% <86.66%> (-0.62%)	⬇️
rocketchat/calls/im/get_history.py	90.9% <90.9%> (ø)
rocketchat/calls/im/create_room.py	90.9% <90.9%> (ø)
rocketchat/calls/im/get_rooms.py	91.66% <91.66%> (ø)
... and 1 more

---

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[pipozzz]

Looks good to me !

[pipozzz]

Thank you very much for contribution !