add support for api/v2 endpoints, enable retry

[alpinweis]

• support multiple versions of the API
• add methods for api/v2 endpoints: pipeline/:id, pipeline/:id/config, workflow/:id, workflow/:id/jobs, project/:slug
• enable retrying for failed requests (5xx)
• add shallow param to get recent builds

[prasket]

Looking forward to this PR. Just found this project and was sad to see it didnt support v2 until I saw this. :)

[alpinweis]

any interest in reviewing and moving forward with this PR ?

[levlaz]

@alpinweis I really appreciate your patience here. I am going to review this again and hopefully get it merged in this week.