Small changes

[pmetras]

This pull request addresses 4 small points:

• Retrieve the mtime of JSON file only if file exists. The scanner could not run if the cache was empty.
• Explained the algorithm used to retrieve user defined metadata in album.ini in documentation.
• Tagged Python 2 specific code with # @python2 comments so that we can delete it from the source in the near future.
• Corrected the bug of the user defined metadata not found with Python 2 when filename contains accented characters (issue #79)

[paolobenve]

thank you!