Color lightness adjustments using scale-color instead of lighten/darken

As discussed in issue #3749, lighten/darken can easily push colors into pure white or black. At @mhayes request, I makes those changes, replacing all occurrences of lighten and darken with scale-color. Play around and see if you like it better of worse than lighten/darken.

* I have left styles in doc/assets/scss/ alone.
* I have changed the percentages and function-factors to produce identical results as the pre-existing values had with the *old* color methods.
* In some cases (say 1/10 color variables), rounding errors have pushed colors by 1 rgb value. E.g. f08a24 might have changed to f18b25 at worst case. Another way of saying it is that I have not used fractional percentages to push color values exactly right.
* In cases where the documentation "examples" did not match the sass code I assumed the code to be correct and made the required changes to the docs.
* Grunt does not complain but we'll see what Travis has to say. :)


Note: I did find documentation I did not correct. The variables for "section" seem to have been removed from the framework but still have documentation in examples_sass_variables.html. Search for $section to have a look.