Update JS styleguide.

Problem description
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The JS styleguide is still young and it requires expansion, at least for comments conventions.


Changes description
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Added guides for comments and global variables.


How to try and test the changes
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reviewers @alibotean @bnickname 

I have also updated the website http://styleguide.chevah.com/javascript.html

Check that changes make sense and you are happy with them.

Please use this review to add any other comment related to JS styleguide.

Note that styleguide , even if is part of Chevah project, it is released as an independent open project. Tickets are not listed in our internal Trac.
 In this way other teams can check and reuse it.