Reworking the API for assert_style to be as discussed on pull request.

asserts that elements returned from a query selector have a list of styles.

Assert the element with id #hello is 100px wide;
    assert_styles(document.getElementById('hello'), {'width': '100px'})
    assert_styles('#hello'), {'width': '100px'})

Assert all divs are 100px wide;
    assert_styles(document.getElementsByTagName('div'), {'width': '100px'})
    assert_styles('div', {'width': '100px'})

Assert all objects with class "red" are 100px wide;
    assert_styles(document.getElementsByClassName('red'), {'width': '100px'})
    assert_styles('.red', {'width': '100px'})

Assert first div is 100px wide, second div is 200px wide;
    assert_styles(document.getElementsByTagName('div'),
                  [{'width': '100px'}, {'width': '200px'}])
    assert_styles('div',
                  [{'width': '100px'}, {'width': '200px'}])