This will allow std::string to be used with vtkSet/Get macros.

See discussion thread

 VTK coding conventions clarification
 http://www.vtk.org/pipermail/vtk-developers/2014-March/014867.html



It is not possible to portably forward-declare std::string or the
std::basic_string<> template, but we do not want to include <string>.
Instead, use a template that happens to match std::basic_string<> and
forwards to a private implementation (where <string> and <iostream>
are available) by casting through an internal concrete type.  Use
std::char_traits<> and std::allocator<> declarations that we know
vtkSystemIncludes.h brought in before including the
vtkOStreamWrapper.h header.

Add a new "TestOStreamWrapper" test case to cover use of the
vtkOStreamWrapper with std::string in a translation unit that acts
like wrapper-generated sources (define VTK_STREAMS_FWD_ONLY) where
no <iostream> is included.

Suggested-by: David C. Lonie <david.lonie@kitware.com>
Change-Id: I5bf4803b0e4b1e7c899b061fdbd6f66f08cbc7d6