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.

src/OpenFOAM/db/error/messageStream.C

@@ -31,17 +31,18 @@ License
 
 int Foam::messageStream::level(Foam::debug::debugSwitch("level", 2));
 
-// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+// * * * * * * * * * * * * * * * * Constructors  * * * * * * * * * * * * * * //
 
 Foam::messageStream::messageStream
 (
     const string& title,
-    errorSeverity sev,
+    const errorSeverity severity,
     const int maxErrors
 )
 :
     title_(title),
-    severity_(sev),
+    severity_(severity),
     maxErrors_(maxErrors),
     errorCount_(0)
 {}
@@ -56,6 +57,8 @@ Foam::messageStream::messageStream(const dictionary& dict)
 {}
 
 
+// * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //
+
 Foam::OSstream& Foam::messageStream::masterStream(const label communicator)
 {
     if (UPstream::warnComm != -1 && communicator != UPstream::warnComm)
@@ -76,6 +79,8 @@ Foam::OSstream& Foam::messageStream::masterStream(const label communicator)
 }
 
 
+// * * * * * * * * * * * * * * * Member Operators  * * * * * * * * * * * * * //
+
 Foam::OSstream& Foam::messageStream::operator()
 (
     const char* functionName,
@@ -237,7 +242,7 @@ Foam::messageStream::operator Foam::OSstream&()
 }
 
 
-// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+// * * * * * * * * * * * * * * * Global Variables  * * * * * * * * * * * * * //
 
 Foam::messageStream Foam::SeriousError
 (