2. Improved file_helper::exists() performance under linux to use stat sys call
3. Added unit tests

Fixed missing #pragma once

Fixed shadow warning in gcc

spurious character broke compilation

Replace GetDynamicTimeZoneInformation to GetTimeZoneInformation.

Renamed "Windows.h" to "windows.h" for building spdlog on MinGW using case-sensitive file system.

Update syslog_sink.h

Syslog already adds formatting, such as the identifier set with `openlog` in the constructor, the priority, and timestamp -- using the formatted message duplicates this information in the log message. This especially causes problems when the syslog is forwarded to aggregators such as Loggly, Logstash, etc. which can parse log messages which are JSON. However, the duplicated fields which spdlog prepends interfere with this -- better to use `raw` in the syslog case I think, or perhaps add an ability to use custom formatters on individual sinks.

Add distributional sink

Setting up dist_sink to a logger will inject a layer between the logger and
(some of) it's sinks. Sinks can be added and removed to dist_sink dynamically.

Fix misprint

Removed redundant check

Add Android sink