Fixed header include problems.

Allow syslog logger on Mac OS X

use default move constructor/assignment for async_log_helper

Fixed issues with time zone UTC offset and daylight saving offset on Windows

- fixed false error (returning -1) in time zones without daylight saving (checking against TIME_ZONE_ID_INVALID instead of 0)
- accounts for daylight saving only when tm::tm_isdst is true
- accounts for standard time offset ([DYNAMIC_]TIME_ZONE_INFORMATION::StandardBias) in time zones that need it

Add zf_log to performance tests

On my machine I'm getting following results (real time, best of 3):
        1 thread   10 threads   100 threads
zf_log   0.466s     2.711s       2.608s
spdlog   0.747s     5.121s       5.376s

Added CLOCK_MONOTONIC trick

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