Added binary-tree layout orientation [fixes #13]

This is an attempt at allowing for a binary-tree orientation method requested in Issue #13.

This is the bare minimum for implementing this feature, however, it seems a bit silly to use two different layout methods for vertical/horizontal and binary tree. I think it would be better to use the `layout` for all of them, but that might be overstepping the bounds of this pull request. Anyway, looking for feedback before I continue.