fix URLConverter bug

In less background url:

    background: url("../image/a.jpg") no-repeat scroll 0 0 transparent;

source_dir:

    http://www.a.com/static/css/

`convert` method (in `utils.URLConverter`) convert them to :

    'http:/www.a.com/static/image/a.jpg'

'http:/' is wrong.

'urlparse.urljoin' should be used at this time:

    urlparse.urljoin('http://www.a.com/static/css/', '../image/a.jpg')
    >>> 'http://www.a.com/static/image/a.jpg'
    urlparse.urljoin('/static/css/', '../image/a.jpg')
    >>> '/static/image/a.jpg'