Allow to get recent builds of a branch

As for
https://circleci.com/docs/api#-recent-builds-for-a-single-project-recent
-builds-project-, you can specify the branch name as a filter.

The method `getBranchBuilds` works the same way as `getBuilds`, but
appending the branch name to the URL
[getBranchBuilds] Added about new method to README
Merge pull request #2 from iambrosi/branch-builds

Allow to get recent builds of a branch