Changed the rounding constant values to match their variable names.

Changed the rounding constant values to match their variable names.
This serves to made the context representation more useful (the names in
match the names out).

Removed the link to ReXX.  That document did not shed add anything to
information already in the spec.

Further tighten the public API to improve usability:

* Emax and Emin are set through Context().  Their defaults are set in the
  DefaultContext, so there is no need to expose DEFAULT_MAX_EXPONENT
  and DEFAULT_MIN_EXPONENT.

* The string functions isnan() and isinfinity() were only used internal to
  the module and were not among the test cases.  External use cases are
  served by the Decimal constructor.  So, made these private.