Added Helper CSS

[phoenixh]

Added Helper CSS with Display and Hide Pure-U Blocks with Viewport.
Added Text Alignment class

[phoenixh]

added the offset of cols

[superscott]

+1 for formalized offset class.

[ericf]

@phoenixh this adds a few things in one PR which I think are separate discussions — in fact there's already threads on most of these things: #326, #318

In general we want to avoid adding things to Pure that people can easily do in their app's CSS; e.g., .text-left is not something we'll ever add to Pure, because it can be easily done in the app's code base.

[ericf]

I don't understand the point of these, and using the word "display" to mean the absence of displaying is confusing.

[phoenixh]

@ericf as it is a helper class and some usefull classes are in that file i think it may be a good additional css file to use in the pure FW.

[ericf]

@phoenixh Thanks again for creating these helpers. As I noted above, This is not something we want to add to Pure's core. But you should feel free to publish this as an extension that people can use with Pure.

[bchenSyd]

'I don't understand the point of these, and using the word "display" to mean the absence of displaying is confusing.'
it's meant to be overridden later. e.g.

@media (max-width: 767px) {
     .display-sm {
      display: block !important;
  }
}

so if you have a <div class='display-sm' >, that <div> is only visible in small screen.

I think this is the same as in bootstrap, almost a standard way to do that