Utility: std::fstream accepts std::string in C++11.

Finally.
Utility: don't use platform-dependent API for checking file existence.

We have Directory::fileExists() wrapper already, use that instead. Also
cleaned up the code a bit.
Utility: blind proper port of Directory::mkpath() and list() to Windows.

Now it should be usable without MinGW. I don't know whether this works,
but at least it compiles :-)
Removed redundant CORRADE_*_EXPORT macros.

They are not needed (ignored by GCC and Clang, cause errors on MSVC).