Add support for other REST APIs

This is a terrific library, but when I looked into it closely, I realized that it needed some significant refactoring to support the branch-permissions API which was a hard requirement for me to use it.  I have done one version of that refactoring.  It includes changing the existing "core" api calls from stashy to stashy.core (see changes in readme).  I realize that this is a significant departure, and would welcome feedback from project maintainers.