Clean includes and usings

These changes are based on suggestions made by the include-what-you-use
tool. Some suggestions were not implemented because they did not
decrease build time (forward-declaring some classes), or because they
would make the include section too complicated (Spirit headers and MPL
headers). On my computer, this change decreases the build time by about
27%:

before
real	1m45.807s
user	5m40.648s
sys	0m18.274s

after
real	1m18.013s
user	3m57.866s
sys	0m12.946s

Change-Id: I11e4f30f4e7518506a213537daae9c41d52b4ac4
diff --git a/src/interpreter.cpp b/src/interpreter.cpp
index d9d5214..b469149 100644
--- a/src/interpreter.cpp
+++ b/src/interpreter.cpp
@@ -6,9 +6,11 @@
  *
 */
 
+#include <boost/mpl/for_each.hpp>
 #include <iostream>
 #include "datastore_access.hpp"
 #include "interpreter.hpp"
+#include "utils.hpp"
 
 void Interpreter::operator()(const commit_&) const
 {