mathMod

The examples were meant just as demonstrations; in this case to clarify the difference between `mathMod` and `mod`.  doctest looks quite interesting, as does [autodoc][ad], which @megawac  mentioned in [#169][doc], but I would really rather get our feet under us with the documentation before I spent any time thinking about extending it to anything further.


  [ad]: https://github.com/dtao/autodoc
  [doc]: https://github.com/CrossEye/ramda/issues/169#issuecomment-50108169