Replace boost fs with native fs

[jutanke]

To remove dependencies

[nebw]

see #25

[nebw]

we have to determine the required gcc/clang/msvc versions for c++14 filesystem support.

[nebw]

msvc implemented filesystem in 2015

[hmoenck]

As far as I can tell we are only using filesystem, optional, ...? std is having those as well. Having it build with MSCV 2017 would be nowadays.

[nebw]

std::filesystem is a C++17 feature

[hmoenck]

Talk to you again in 3 years or replace filesystem by some manual approach? I forgot we've been talking about this being a 17 feature...

[nebw]

#include <experimental/filesystem> // C++-standard header file name  
#include <filesystem> // Microsoft-specific implementation header file name  
using namespace std::experimental::filesystem::v1;  

If you don't mind this somewhat ugly include block, you can use std::filesystem with MSVC >= 2015.

https://msdn.microsoft.com/en-us/library/hh874694.aspx