Positioning items dropdown as 'fixed' so that it can overlap containers...

...with hidden overflow

@grahamscott: Please review. 

Note that this PR will conflict with PR #20 as that change removes the searchContainer from the .items element (which means it won't be included in the fixed positioned container). 

I suggest we keep the .items element as the "wrapper" for everything that should be displayed in the dropdown (similar to how it's currently done for the MULTI template). The SINGLE template could look something like this:

    ...
    '<div class="items">' +
        '{{#showSearch}}' +
        '<div class="searchContainer">' +
            '<input class="search"></input>' +
        '</div>' +
        '{{/showSearch}}' +
        '<ul class="items">' +
            '{{#items}}' +
            '<li class="item" data-text="{{label}}" data-value="{{value}}">' +
                '<span class="itemText">{{label}}</span>' +
            '</li>' +
            '{{/items}}' +
        '</ul>' +
    '</div>' +
    ...