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/test/router/Test-processorRouter.C

@@ -28,10 +28,8 @@ Description
 #include "argList.H"
 #include "label.H"
 #include "labelList.H"
-#include "OStringStream.H"
-#include "IStringStream.H"
-#include "OFstream.H"
 #include "IFstream.H"
+#include "OFstream.H"
 #include "point.H"
 #include "Time.H"
 #include "fvMesh.H"