Add the SPDLOG_USE_WCHAR tweak to enable support for Unicode names on Windows.

[Kentzo]

Refs #111

[gabime]

I reverted. didn't compile properly under vs2015

[Kentzo]

@gabime What was the error?

I'll install vs2015 and test later today.

[gabime]

an #include <codecvt> was missing in common.h

I will make some changes and merge again soon

[Kentzo]

@gabime This include caused errors in tests. See https://travis-ci.org/gabime/spdlog/jobs/120374514

We need to find a fallback method for turning wstring to string that should be executed in case compiler does not support codecvt.

The codecvt version should be preferred though.

[gabime]

right. seems like gcc don't have this file.

[gabime]

I think I will add something like

#if defined(_WIN32) && defined(SPDLOG_USE_WCHAR)
#include <locale>
#include <codecvt>
#endif

[Kentzo]

@gabime It's rather libstdc++. But I read somewhere that modern versions do support it.

Not sure how could we figure this out exactly, but your change seems sane.

[Kentzo]

@gabime Did this fix work for you?

[gabime]

@Kentzo sorry, I did not have time to get to it.

[Kentzo]

@gabime See #193