hidden-md never shows up

[MitchTalmadge]

Using <b:column colMd="0">, hidden-md never appears in the classes. In fact, quite the opposite happens, and col-md-12 shows up. (Only if colMd is the only attribute. If colXs is specified, for example, then col-xs-# will appear and not col-md-12.) xs, sm, and lg work fine.

[stephanrauh]

It seems setting using zero is an undocumented feature. I fixed the bug nonetheless. I just hope I didn't break too many programs relying on the bug :(.

BTW, the additional col-md-12 seems to be important. It's added to every size, not only md. So I added it to `col-md="0"´, too.

[stephanrauh]

@TheCoder4eu @asterd Please have a look at my bugfix. I want to be sure I didn't break anything. Thanks!

BTW, I've documented these features at the basic.xhtml.