Add logging utility which prints messages to standard output synchronously.

[kowalej]

• Logging utility class with mutex to prevent multiple threads from writing ugly run-on messages in the console.
• Log functions for various levels (info, warning, or error) which can be hooked into for future logging (text file logging or otherwise).

[tstenner]

All log methods have the same signature and contents, so I'd rather have a single log function that takes the level as parameter. std_out is unused, so I'd remove it. It's a header only inline function so we can add a default parameter later anyway

[kowalej]

I agree with removing currently uneeded stdout flag param. However I think it's better for usability to keep the levels as different functions. This "function per log level" is pretty standard in logging libraries.