ENH: make treatment of stream allocators more uniform (issue #532)

- use allocator class to wrap the stream pointers instead of passing
  them into ISstream, OSstream and using a dynamic cast to delete
  then. This is especially important if we will have a bidirectional
  stream (can't delete twice!).

STYLE:

- file stream constructors with std::string (C++11)

- for rewind, explicit about in|out direction. This is not currently
  important, but avoids surprises with any future bidirectional access.

- combined string streams in StringStream.H header.
  Similar to <sstream> include that has both input and output string
  streams.

applications/utilities/surface/surfaceMeshConvertTesting/surfaceMeshConvertTesting.C

@@ -71,8 +71,7 @@ Note
 #include "ModifiableMeshedSurface.H"
 #include "UnsortedMeshedSurfaces.H"
 
-#include "IStringStream.H"
-#include "OStringStream.H"
+#include "StringStream.H"
 
 using namespace Foam;