Bump minimum Boost ver to 1.54 for Boost.Log.
Depend on Boost.Log.
Initial Boost.Log with varargs.
Move detail::Fmt to impl and fill out levels.
Re-route SDL_Log to Boost.Log.
Set the filter for Boost.Log.
Set the Windows debugger sink.
Explain why we don't use debug_output_backend.
Fix Boost.Log dynamic linking.

Some distributions of Boost.Log, including Debian's, have linking
problems.  Specifically defining BOOST_LOG_DYN_LINK fixes it.
Define BOOST_ALL_DYN_LINK.

This is cleaner than just BOOST_LOG_DYN_LINK.
Add logging to std::clog.
Move init of sinks into separate functions.

Also removed unused include.
Fix output debug log sink init.
Include severity in log entries.
Fix indentation.
Notify event listeners when log entries are added.
Use narrow chars for stream backend.
Clean up unused params.
Add Boost.Log's trivial logger to PCH.
Align Log::Level and Boost.Log's trivial logger.