Implement the Git methods to pull, add, commit and push the dates

[raduiman-zz]

This PR will:

Add/optimize the following methods:

• def pull(self) -> updates the active local branch from remotes/origin
• def add(self) -> add the changed files from the following paths: [CHANGELOG_JSON_PATH, CHANGELOG_MD_PATH, CHANGELOG_REPO_PATH]
• def check_for_changes(self) -> it checks if there are changes staged for commit
• def commit(self) -> commit the changes. Commit messaged: Changelog: datetime.utcnow()

• def push(self) -> push the commit to active remotes.origin branch

  The Logs from test:

• [05:39:33] [FIC_Github.py:pull:30] pulling changes from https://github.com/mozilla-releng/firefox-infra-changelog.git -> Branch riman
• [05:39:34] [FIC_Github.py:push:50] Summary of pull: FETCH_HEAD
• [05:39:35] [FIC_Github.py:commit:46] Committing changes with message: Changelog: 2019-04-14 02:39:35.008281
• [05:39:35] [FIC_Github.py:push:52] Summary of commit da48888785386125a059258059ffca2fe4347b72
• [05:39:35] [FIC_Github.py:push:53] pushing changes to https://github.com/mozilla-releng/firefox-infra-changelog.git on branch riman

• [05:39:40] [FIC_Github.py:push:54] Summary of push: 80e275e..da48888

  Other necessary additions:

• added changelog.json and changelog.md files

[raduiman-zz]

Any specific reason for why 'GitError' and the Try-Except part was removed?

I need to find out more about how to use "GitError" and how to implement it properly in our code. any help is welcome