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/HashTable/Test-hashTable.C

@@ -29,8 +29,7 @@ License
 #include "DynamicList.H"
 #include "FlatOutput.H"
 #include "IOstreams.H"
-#include "IStringStream.H"
-#include "OStringStream.H"
+#include "StringStream.H"
 
 using namespace Foam;