Improve word boundary detection in non-english charsets

This is an implementation of the fallback solution identified in https://github.com/kogmbh/WebODF/issues/268#issuecomment-34034988.

# TODO
- [x] More unit tests around rounding when not on an existing word boundary
- [x] Update some API docs
- [x] Figure out how to fix coverage :-/ (Yay!. Removing the "〾" character makes things happy again. This equivalent to \u303F)