Bitbucket support

[jihonrado]

This library works only with Github with the current implementation, as it uses URLs like this:

https://circleci.com/api/v1.1/project/:username/:project/tree/:branch

Following CircleCI documentation, the VCS can be specified by using URLs like:

https://circleci.com/api/v1.1/project/:vcs-type/:username/:project/tree/:branch

Not providing the vcs-type path parameter makes CircleCI consider github as VCS.