Bitbucket cloud support

[MarekFort]

Please consider adding support for Bitbucket status update. The API for Bitbucket is really similar to the one for Stash.

Bitbucket API docs: https://confluence.atlassian.com/bitbucket/buildstatus-resource-779295267.html

[scaytrase]

Bitbucket Cloud may involve another authentication mechanism (OAuth) and use another API endpoints, which currently are not consistent with Bitbucket Server API.

Currently Bitbucket support requires a bunch of refactoring passes but I'll take this into account, but no promise to make it soon

[tstibbs]

Note that https://github.com/jenkinsci/bitbucket-build-status-notifier-plugin exists for BitBucket Cloud.

[scaytrase]

I think now it's possible to extend ideas from this PR #273 in order to achieve different clients for different versions of API