fixed bug that make impossible to use the linter if gutter_marks are disabled. Fixes #23
small refactor in server.py to follow PEP-8
total refactor of sublime_python_linting.py file:

    * Fixed several PEP8 violations
    * Made the code more DRY
    * Added some decorator
    * Some big methods has been splitted in serveral methods
    * Changes are been tested and them are working fine
some PEP-8 violation fixed on server/linter.py
merged