fix compilation with boost::filesystem

CMakeLists.txt:
---------------------------
Until now build with boost::filesystem was completely broken,
since cmake exported definitions with wrong name prefixes, and
nana always defaulted to internal filesystem implementation.

After fixing the boost definitions, a number of errors came up
due to incompatibility of boost::filesystem with nana and std
filesystems. This commit tries to fix them all.

filesystem.cpp, filesystem.hpp, filebox.cpp:
--------------------------------------------
boost::filesystem doesn't have a file_time_type, so declared
it in the filesystem.hpp header.

boost::filesystem::last_write_time has a return type std::time_t
unlike the other two implementations of this function in nana
and std, so added ifdef to convert the result to file_time_type.

fixed build on gcc-4.9, since it doesn't have a std::put_time
function, included <nana/stdc++.hpp> in that case.

boost::filesystem::file_type types have different names than
std::experimental::filesystem::file_type types, fixed it by
creating an enum class file_type with the same type names as
in std::experimental::filesystem::file_type. This fix
requires static_cast from functions results to internal file_type,
since boost file_type and std file_type a different enum classes.

changed switch to if, bacause old gcc fails on converting
enum class members to int.

stdc++.hpp:
-----------
added ifndef guards to prevent errors on multiple includes of this header.

wvl.cpp:
--------
added boost/chrono.hpp include for the cases when std::thread is not
available.

travis:
-------
added boost system, thread, chrono libs to install, they are needed
for the nana-demo to compile.

source/filesystem/filesystem.cpp

@@ -14,7 +14,13 @@
 #include <nana/filesystem/filesystem_ext.hpp>
 #include <vector>
 #include <sstream>
-#include <iomanip>	//put_time
+
+#include <nana/config.hpp>
+#ifdef _enable_std_put_time
+	#include <nana/stdc++.hpp>
+#else
+	#include <iomanip>
+#endif
 
 #if defined(NANA_WINDOWS)
     #include <windows.h>
@@ -97,7 +103,13 @@ namespace nana
 		std::string pretty_file_date(const fs::path& path) // todo: move to .cpp
 		{
 			try {
+#if NANA_USING_BOOST_FILESYSTEM
+				// The return type of boost::filesystem::last_write_time isn't
+				// the same as in nana and std implementations of this function
+				auto ftime = std::chrono::system_clock::from_time_t(fs::last_write_time(path));
+#else
 				auto ftime = fs::last_write_time(path);
+#endif
 
 				// crash: VS2015 will not read the time for some files (for example: C:/hiberfil.sys)
 				//   and will return file_time_type(-1) without throwing