Use python logging instead of custom class.

[cmutel]

Fixes #39.

Several clear advantages:

• Don't have to pass custom logger class to transformation functions
• Separation of previous Logger class into OutputDir (which creates the output directory), and create_log, which creates the log.
• Standard logging allows for plugging into library functions and package ecosystem, e.g. shipping logs.

Plus there is generally less magic.

Docs are updated, tests pass on my machine and CI.

[cmutel]

@ecoinvent Please check and merge if you are fine with these changes.

[coveralls]

Coverage increased (+0.2%) to 61.79% when pulling fc1d379 on real-logging into b03bbbd on master.