Added missing grid make-column mixins and fixed some inconsistencies

make-column() was still been use to generate "medium size" columns (col-sm), while make-small-column was creating the default columns (col).
I also changed @grid-float-breakpoint to @screen-small, as it was used in make-row() and I think it makes more sense here than the other.
Merge branch 'patch-12' of https://github.com/ggam/bootstrap into ggam-patch-12
rename and refactor a smidge to add medium grid mixins