New algorithms

Hi all,

I have been accumulating new code for quite some time. Now, it is ready to be submitted. Find in this PR some various algorithms, all implemented in Lua, with tests. 
Here is the complete list:

* [Comb sort](http://en.wikipedia.org/wiki/Comb_sort)
* [ROT47 ](http://en.wikipedia.org/wiki/Rot47#Variants)
* [Various statistical mean computation algorithms](http://en.wikipedia.org/wiki/Mean)
* [Ackermann function](http://en.wikipedia.org/wiki/Ackermann_function)
* [Kadanes algorithm](http://en.wikipedia.org/wiki/Maximum_subarray_problem)
* [Tokenization](http://en.wikipedia.org/wiki/Tokenization)
* [Symmetric difference](http://en.wikipedia.org/wiki/Symmetric_difference)
* [The Twelve Days Of Xmas song](http://www.lyricsmode.com/lyrics/c/christmas_carols/the_twelve_days_of_christmas.html)
* [Self-Descriptive numbers](http://en.wikipedia.org/wiki/Self-descriptive_number)
* [Roman numerals encoding/decoding algorithm](http://en.wikipedia.org/wiki/Roman_numerals)
* [Josephus problem](http://en.wikipedia.org/wiki/Josephus_problem)
* [Happy number](http://en.wikipedia.org/wiki/Happy_number)
* [Harshad number](http://en.wikipedia.org/wiki/Harshad_number)

PS/ I also added some fixes (typos and implementations fixes) to some previous implementations of mine.

Regards,
Roland.