[2.0-wip] .span* class definitions in .row-fluid should be limited by child selector

Created by: reefdog

The .span* default column definitions for .row-fluid in mixins.less should be limited by the > child selector. Otherwise, any .span*'s inside a .row-fluid – even those way down the DOM – are sized improperly.

In other words, this markup generates funky-looking form elements in the sidebar:

<div class="container-fluid">
    <div class="row-fluid">
        <div class="span2">
            <form>
                <fieldset>
                    <legend>Sidebar form</legend>
                    <div class="control-group">
                        <label class="control-label">Name</label>
                        <div class="controls">
                            <input type="text" class="span2"> <!-- This is way too narrow because it inherits the ".row-fluid .span2" sizing  -->
                        </div>
                    </div>
                </fieldset>
            </form>
        </div>
        <div class="span10">
        </div>
    </div>
</div>