Boardwalk CLI: Use python logger; have debug logs

[m4wh6k]

What and why?

This fixes #7 by using a logger for most log messages emitted by boardwalk. It also adds a new global CLI option and env variable to enable debug logging. Usage of ClickException is replaced by a new general "BoardwalkException" so that a logger can be used along with exception messages.

There are a couple known issues coming out of this:

• The debug output is very verbose and not meant for typical usage. It's easy to miss prompts from the CLI because of ongoing messages from background activity.
• The default log format is meant to give the user some idea of where in the application a log message is being emitted. At the moment, the BoardwalkException output doesn't really give the user a better idea of where an error occurred unless another log message preceded it.

How was this tested?

Tested locally using the test Boardwalkfile.py. Also some spot checking with a live workspace.

Checklist

• Have you updated the VERSION file (if applicable)?

[asullivan-blze]

LGTM!