Adding a blank list option.

To go along with the .circle, .square, .disc classes for an un-ordered list a .blank option for those times where you want the list items to be indented but do not want a list-style applied.

This can also be added to the Gumby UI page with the other list examples:
<div class="four columns">
    <h4 class="lead">ul.blank</h4>
    <ul class="blank">
        <li>List item</li>
        <li>List item</li>
        <li>List item</li>
            <ul class="blank">
                <li>Nested item</li>
                <li>Nested item</li>
            </ul>
        <li>List item</li>
        <li>List item</li>
        <li>List item</li>
        <li>List item</li>
    </ul>
</div>