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.

CMakeLists.txt

@@ -238,7 +238,7 @@ elseif (NANA_CMAKE_STD_FILESYSTEM_FORCE)
     add_definitions(-DSTD_FILESYSTEM_FORCE)
 elseif (NANA_CMAKE_FIND_BOOST_FILESYSTEM OR NANA_CMAKE_BOOST_FILESYSTEM_FORCE)
     if (NANA_CMAKE_BOOST_FILESYSTEM_FORCE)
-        add_definitions(-DNANA_BOOST_FILESYSTEM_FORCE)
+        add_definitions(-DBOOST_FILESYSTEM_FORCE)
     endif(NANA_CMAKE_BOOST_FILESYSTEM_FORCE)
     # https://cmake.org/cmake/help/git-master/module/FindBoost.html
     # Implicit dependencies such as Boost::filesystem requiring Boost::system will be automatically detected and satisfied,
@@ -246,7 +246,7 @@ elseif (NANA_CMAKE_FIND_BOOST_FILESYSTEM OR NANA_CMAKE_BOOST_FILESYSTEM_FORCE)
     # If using Boost::thread, then Thread::Thread will also be added automatically.
     find_package(Boost COMPONENTS filesystem)
     if (Boost_FOUND)
-        add_definitions(-DNANA_BOOST_FILESYSTEM_AVAILABLE)
+        add_definitions(-DBOOST_FILESYSTEM_AVAILABLE)
         include_directories(SYSTEM "${Boost_INCLUDE_DIR}")
         list(APPEND NANA_LINKS ${Boost_LIBRARIES})    ######  FIRST !!!!!!!!!!!!!!!!!    add   is not first
     endif (Boost_FOUND)