#30 Code style enhancement

[Pietro Zambelli]

I've tryied to modify the istsoslib/filters code trying to follow this main idea:

• follow the PEP8 directive (I didn't changed class' and function's name);
• reduce the line length at least to <100 characters;
• where possible reduce the number of line;
• reduce the indentation level.
• substitute if/elif/chain using dictionary.
• substitute string like "something \"else\"!" with 'something "else"!'