Refactoring grid implementation.

This is a "preview" PR for discussion and feedback. Since there're several possible methods each with its own pros and cons I had to write a small [(pseudo)-blog-post about this](https://github.com/seven-phases-max/less.curious/blob/master/articles/rbgi.md). Please read.

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(**Updated**) This PR implements **`Method #1`**. The generated CSS should be equal to what it is now, except 5 added dummy classes (`.col-any-any`, `.col-xs-any`, `.col-sm-any`, `.col-md-any`, `.col-lg-any`) plus the generated grid classes are (intentionally) reordered. (Note also that all mixins are rewritten and reordered so the `diff` view does not make any sense).

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A few questions. I'm not sure if "coding guidelines" questions (specifically "What Less features may we use?") are appropriate here but since this directly affects the code in the PR I guess I'll ask. In particular:
 - [Pattern Matching](http://lesscss.org/features/#mixins-parametric-feature-pattern-matching). I can see that `when` guards are *always* used even where the "Pattern-Matching" would result in more clean and less verbose code: is there any special reason behind this?
 - Temporary variables. I also was not able to find (though I could just miss) even a single temporary variable (specifically a temporary variable inside a mixin) in the sources: is there any special reason for this?
 
Thanks.