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

I encountered an issue when using spdlog through add_subdirectory.
Since SPDLOG_BUILD_TESTING is ON by default, it now adds tests to my project, even if BUILD_TESTING (the official CTest variable) is set to OFF.
cmake_dependent_option makes it so that if someone enables BUILD_TESTING then SPDLOG_BUILD_TESTING will be set to ON by default.
This way one can disable all external tests by setting BUILD_TESTING before using add_subdirectory and then setting it back to its original value.
The only change for those using the library directly is that they now use BUILD_TESTING instead of SPDLOG_BUILD_TESTING when configuring.

Issue-488: warning for missing enumeration value

readme: Add Gentoo package name

Background color unchanged

Keep the background color unchaged on Windows console when using colored loggers.

Added TRACE/DEBUG_IF macro definitions when TRACE/DEBUG_ON aren't defined