Create a simple logger for output

[rboston628]

Thrain previously had written all output directly to stdout using printf. This made it hard to silence or redirect output.

This creates a simple logger with four levels, and updates the program to write with this instead printf. Output is written almost identically to how printf works, including a format string with variadic arguments.

A set of comprehensive tests ensures the logger works as expected.

It is in principle possible to silence all output or to redirect it to a local file instead of the screen. A future update should add that feature to the command line args.