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

src/lib_json/json_writer.cpp

@@ -11,7 +11,6 @@
 #include <assert.h>
 #include <stdio.h>
 #include <string.h>
-#include <iostream>
 #include <sstream>
 #include <iomanip>