Got rid of several unnecessary includes of <iostream>.

Including <iostream> causes the file to be polluted with a static
initializer for the __ioinit symbol. This can harm binary startup time.
For more info, see here:

    http://neugierig.org/software/chromium/notes/2011/08/static-initializers.html

include/json/writer.h

@@ -11,7 +11,6 @@
 #endif // if !defined(JSON_IS_AMALGAMATION)
 # include <vector>
 # include <string>
-# include <iostream>
 
 namespace Json {