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

Update README.md

Updated install section for Arch Linux. Added yaourt instead of pacman.

Support for printf-style logging

* tests updated for printf style
* tests makefile support printf style

Fix SPDLOG_WCHAR_TO_UTF8_SUPPORT wchar_t logging

add formatter for unix epoch time in seconds.

Compiler-dependent line numbering

Feature/improve cmake lists

* support CMake user registry package registration
* use GNUInstallDirs to set installation firectories
* use spdlog namespace in both build and install interfaces

SPDLOG_BUILD_TESTING now depends on BUILD_TESTING