add test with iwyu and fix all headers accordingly

tests/iwyu.py

@@ -0,0 +1,21 @@
+#!/usr/bin/env python
+
+from __future__ import print_function
+
+import json
+import subprocess
+
+with open("compile_commands.json", "r") as f:
+    tunits = json.load(f)
+
+result = 0
+for tu in tunits:
+    _,rest = tu["command"].split(" ",1)
+    # iwyu does not distinguish between different outcomes of its check
+    # so instead, we grep its stderr output
+    # see http://code.google.com/p/include-what-you-use/issues/detail?id=157
+    ret = subprocess.call("%s %s 2>&1 | grep \"has correct #\""%("iwyu", rest), shell=True)
+    if ret != 0:
+        result += 1
+
+exit(result)