OAuth2 implementation and few more things

[akosipc]

Hi @zuzannast,

Thanks for making this gem and I'm submitting some changes and bug fixes I've encountered while using it.

Here are the list of changes I made:

• Brings back the Authorize button if a OAuth2 param was sent over.
• Replaces oauth2_swagger_docs_path to oauth2_swagger_docs_url in the oauth2RedirectUrl. This is so we can have an absolute URL in the redirect URI. I was using the doorkeeper gem and they don't accept URI that are not absolute.
• Changes the dependency from Rails 5 to Rails 4.2. I haven't deploy this to our staging server but it looks okay upon testing in my dev environment.
• Fixes some README.md config stuff.

Also, tagging #17

[zuzannast]

Sorry for the late review and thank you very much for your input. 🎉