Implement GPG keys management

[guyzmo]

• have an API similar to the SSH keys management, Add SSH key upload feature #22
• need implementation from github3.py for github: Implement the gpg_keys API sigmavirus24/github3.py#704
• need implementation from python-gitlab/gitlab: gitlab/gitlab-ce#4232
• need implementations from gogs: Support of PGP signed commits and tags gogs/gogs#2321
• Gitea has an implentation: Implement GPG api go-gitea/gitea#710 ; Implement Gitea GPG API unfoldingWord-dev/python-gitea-client#14
• need implementations from Gitbucket: [Feature] GPG Keys management gitbucket/gitbucket#1589

and why not implement integration keybase.io?

[guyzmo]

it's pretty close to the implementation of #22

[why-Radhika]

Can you assign this issue to me..I would like to work on it as my first issue.