If the user's environment didn't support unicode, we used
to fall back to using the AsciiTreeGuide if LineTreeGuide
was being used (which it is by default). This commit removes
that fallback since the characters used in LineTreeGuide is
covered by extended ASCII, which SHOULD be fine in almost all
scenarios.
Closes#324
Fixes some tree rendering problems where lines were not properly drawn
at some levels during some circumstances.
* Change the API back to only allow one root.
* Now uses a stack based approach to rendering instead of recursion.
* Removes the need for measuring the whole tree in advance.
Leave this up to each child to render.
