Rename .col-xs to .col + some other cleanup

[Starsam80]

I changed lots of little stuff but the main points are

• Use breakpoint-min instead of a counter: This cleans up the code quite a bit and makes it more readable.
• Add > .col, to the .no-gutters class. Without this, > [class*="col-"] wouldn't match .col, and changing it to something like > [class*="col"] would make it match .collapse and more.
• Rename .col-xs and .col-xs-auto to .col and .col-auto
• Update some documentation to use new classes that weren't changed before
• Fixed a minor bug where even though $enable-flex was false, it would still produce parts of flex classes.

[mdo]

<3<3<3

[einfallstoll]

Wouldn't it make more sense to come up with a table like this:

Name	Tiny*	Extra small	Small	Medium	Large
Size	<576px	≥576px	≥768px	≥992px	≥1200px
Prefix	.col-	.col-xs-	.col-sm-	.col-md-	.col-lg-

* I actually don't have a better name to come up with. (Does it even make sense to be that small? I mean my smartphone is huge)

This would be backward-compatible while still implementing this "tiny .col-"-class.

And another question: Do you really call ≥1200px "extra large"? This ain't half of my screen. Extra large is more like ≥1400px or ≥1500px to me.