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;`

Allow compilation on platforms with unwind (android)

Based on review feedback.

Adding additional build environments for AppVeyor

Fix typos. Thanks @berkus 

rethrow unwind exception

On Linux with pthread library spdlog causes an SIGABORT and crashes
the application in case it catches a thread specific cancellation
exception in a critical execution phase while in a try/catch block
in spdlog/detail/logger_impl.h

The exception is caught by some general catch(...) clause where
it is NOT rethrown.

However rethrowing these kind of exception is mandatory, otherwise
an abort will be caused by the glibc.

Fix warnings which are caused by C style cast