This should supersede revision e3ec6b17260c (but please test before removing that).

Patch by David Edelsohn.

Patch by David Edelsohn.

by Steven D'Aprano.

See http://bugs.python.org/issue19293

See http://bugs.python.org/issue19295

- explain single use, reusable and reentrant in docs
- converted suppress to a reentrant class based impl
- converted redirect_stdout to a reusable impl
- moved both suppress and redirect_stdout behind a functional
  facade
- added reentrancy tests for the updated suppress
- added reusability tests for the updated redirect_stdio
- slightly cleaned up an exception from contextmanager