vtkOStreamWrapper: Support std::string

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

Forward-declare std::basic_string<> to avoid including <string>
everywhere.  Use std::allocator<> and std::char_traits<> declarations
that we know vtkSystemIncludes.h brought in before including the
vtkOStreamWrapper.h header.

Inspired-by: David C. Lonie <david.lonie@kitware.com>
Change-Id: I8f4d5a032c4348b270c69bd2e33f64bf708108c5
Merge topic 'core-string' into master

7feab87c vtkOStreamWrapper: Support std::string