Add Gitter.im sensor

[fabaff]

Description:

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#2800

Example entry for configuration.yaml (if applicable):

sensor:
  - platform: gitter
    api_key: aferfSdss80980effsdfafröljlgkjoier793j838fjjfj
    room: home-assistant/hassio

Checklist:

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.github.io

If the code communicates with devices, web services, or third-party tools:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• New dependencies have been added to the REQUIREMENTS variable ([example][ex-requir]).
• New dependencies are only imported inside functions that use them ([example][ex-import]).
• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

[mention-bot]

@fabaff, thanks for your PR! By analyzing the history of the files in this pull request, we identified @balloob, @robbiet480 and @happyleavesaoc to be potential reviewers.

[balloob]

🐬