Replace `print()` statements with logging

[measrainsey]

Simple PR to replace print() statements with python's logging functions.

This should fix #24.

I've implemented a custom logger that:

• creates logs at each module level (__name__)
• sets the logging level at logging.info()
• saves the logs to a file that is the name of the module (I've added .log files to .gitignore)
• adds some additional syntax such as date and time

For example, running run_preprocess.py results in the following log files:

• main.log
• message_ix_buildings.chilled.preprocess.archetypes.log
• message_ix_buildings.chilled.util.util.log

Hopefully a simple review, but please let me know if I should fix anything.