Added the option to authentical using OKTA lib (which use RS256 alg).

[thomasyip]

- Summary This PR adds support for OAuth2 RS256 alg via Okta's golang lib. There were a few typical pitfalls on implementing OAuth, we feel it was better the lib developed by our vendor.

The PR abstracts auth.go to make the new implementation work alongside with the existing alg. It also makes adding other alg or implementing it with other lib easier later.

- Test plan The Docker on master branch when we pulled didn't build and had no instruction on how to get it runs. We got it to built and provided some instructions. We didn't add test in this PR, but believe we improves the quality a little bit over existing condition for the Docker use-case.

We like to add ci test with Docker use-case to make sure it keeps it building...

- Description for the changelog Added the option to authenticate using OKTA lib (which use RS256 alg).

• Updated golang to 1.11 (the original docker was on 1.8 and code didn't build on it)
• Added instructions to run git-gateway to README.md
• Added abstraction to auth.go such that different implementations of auth can be used
• Updated example.env

- A picture of a cute animal (not mandatory but encouraged)

[thomasyip]

Didn't aware of travis-ci was setup. Has been working with Docker the whole time.

Will get it run and reopen.