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/utils.cpp b/src/utils.cpp
index cd1cf4b..d06d35b 100644
--- a/src/utils.cpp
+++ b/src/utils.cpp
@@ -5,8 +5,8 @@
  * Written by Václav Kubernát <kubervac@fit.cvut.cz>
  *
 */
-#include <boost/algorithm/string/erase.hpp>
 #include <boost/algorithm/string/predicate.hpp>
+#include <sstream>
 #include "utils.hpp"
 
 std::string joinPaths(const std::string& prefix, const std::string& suffix)