added facilty for syslog

Removed all *_if functions (trace_if, debug_if, info_if,..) because they are redundant and confusing way to preform if

correct include path for sink/syslog_sink.h

accept msvc_sink on all compilers, add name windebug_sink (fixes #554)

modified path on quoted #includes

Paths pointing to the root of the library where replaced for ones relatives to each file.

For example, inside /include/spdlog/details/file_helper.h:

This will look for os.h in /include/spdlog/details/spdlog/details/ which doesn't exists.

replaced with:

Declare variables as size_t rather than unsigned.

Modify `unsigned front, front1, back;` to `size_t front, front1, back;`