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.
Media query (grid and responsive utilities) overhaul

Grid classes have been changed to account for a fourth set of classes,
meaning we now have XS, S, M, and L options. Specifically, we made the
following changes:

* Renames `.col-` to `.col-xs-`
* Adds `.col-md-` for devices 992px wide and up
* Remaps `.col-lg-` for devices 1200px wide and up

Alongside that, we've updated our media queries to better handle
advanced grid layouts:

* All column classes now kick in when they hit a `min-width`, undoing
the #9128 fix in
https://github.com/twbs/bootstrap/commit/db45a60cc86190a060f0baf0b3961cb
c690f3a65
* All column offsets, pushes, and pulls are scoped to an appropriate
range, meaning a `min-width` and `max-width` (thanks for the idea,
@eratzlaff)

We've also modified the widths of our grid containers for the small
device grid (now a max 720px instead of 728px) and large device grid
(now a max 1140px instead of 1170px) to avoid horizontal scrollbar
problems in Firefox (likely due to box-sizing, which is why we didn't
see it in 2.x).

Similarly, we've updated the responsive ultility classes to match the
new four media query approach of the grid system. That means we've:

* Added new `.visible-xs` and `.hidden-xs` classes
* Reassigns visible and hidden classes for small, medium, and large to
matching grid system media queries
* Updates docs examples to match grid system and account for fourth
utility range

-----

Fixes the following:

* #9135: fourth grid tier
* #9302: undoes previous range change for small grid system, so no need
to update any docs here
* #8755: consistent grid and responsive utilities schemas)
* #9195, #9216, and #9227: no more horizontal scrollbars
Merge branch 'patch-12' of https://github.com/ggam/bootstrap into ggam-patch-12
rename and refactor a smidge to add medium grid mixins
update docs affix for large desktop given latest grid changes
add matchdep to load all grunt tasks
fixes #9317: document media queries
add .gitattributes file

Info:
https://www.kernel.org/pub/software/scm/git/docs/gitattributes.html
fixes #9310: alerts aren't dismissable by default, now requires .alert-dismissable if you need a close button
fixes #8835: make it so active list group items aren't dependent on an <a> tag
fixes #9266 manually on account of list group file reorganization
Merge pull request #43 from yaoazhen/matchdep

add matchdep to load all grunt tasks
Merge pull request #44 from yaoazhen/addGitattributesFile

add .gitattributes file
fix #9239 by restoring old ie8 responsive img foo
Renamed btn-pseudo-states mixin to btn-variant for consistency
Added button-size mixin
Removed remaining Glyphicons mentions in carousel.less
Added Dropdown JS plugin events (fixed #9375)
Aria role search instead of form
Add missing aria-hidden attribs to close buttons