Improve algorithm to check RUT number

[unho]

Previous code tried to apply the algorithm as described in the Wikipedia, and therefore the code was unnecessarily complex because it did the sum from the end of the RUT number, but when coding it is easier to sum from the beginning.

Also:

• Added docstring,
• Added some blank lines to visually separate code blocks and make code more readable,
• Remove unnecesary variable initialization,
• Renamed some variable to have self-descriptive naming instead of cryptic codes,
• Postpone variable creation until they are strictly necessary,
• Put abort earlys at top,
• Remove unnecessary exception checking,
• Return directly the value of the comparison instead of using if..elif, and
• Add constants for the factors instead of creating them on runtime every time the method is called.

All these small changes also allow to significantly reduce code length.