Add `.pure-u-0` and similar classes

[FaultyAstragal]

This code allows for the hiding certain elements based on Media Queries.

[lkraav]

Do you have a demonstration somewhere that shows how this would be used?

[FaultyAstragal]

Not currently, however displaying one layout versus another based on screen size is the point of responsive web design. Showing one div element to mobile users and laying out a different one for desktop users, allows for greater flexibility for designers.

For example:

<div class='pure-u-0 pure-u-lg-1-2'> ... </div>
<div class='pure-u-1 pure-u-lg-0'> ... </div>

This code will display one div element for desktops at half width, and a separate div for mobiles (up to lg size) at full width.

[ericf]

Seems like a good idea to me!

@jamesalley @jconniff @tilomitra what do you all think of this idea?

[jconniff]

Seems helpful, as long as its purpose is documented clearly so people don't think they have to hide and show duplicate code to get responsive layout.

[adelgado]

For the record I have a situation right now where I think this would be the cleanest way (which is how I've been brought to this issue). Is there something missing in this PR to get it merged?

[ericf]

/cc @jamesalley

[teson2000]

+1 for this to be included in standard pure. Why it needs to be inside pure and example here, https://github.com/yahoo/pure/files/854487/purezero.zip (from the thread https://github.com/yahoo/pure/issues/640)