Changes url_query_cleaner to support str or list parameterlist

The other day I found out that ``url_query_cleaner()`` worked passing only a string when you want to remove only one parameter:

    >>> link = 'http://example.com/?page=2&post=hello'
    >>> url_query_cleaner(link, 'post')
        'http://example.com/?post=hello'

This is a mistake according to the current implementation, but it still works because the check with parameter list use Python's **in parameterlist** -- so, if parameterlist is a string, it ends up checking if the parameter is a substring of the parameter given.
This induces the user to mistakes like:

    >>> link = u'http://site.com/?showtopic=42622&view=findpost&p=545745'
    >>> url_query_cleaner(link, 'showtopic')
    u'http://site.com/index.php/?showtopic=42622&p=545745'

Here is a proposal to make the parameter a str or list parameter.

Does this look good?