add ability to add links to legend

>>> import pygal
>>> chart = pygal.Bar()
>>> chart.add('Red', [{
...   'value': 2,
...   'label': 'This is red',
...   'xlink': 'http://en.wikipedia.org/wiki/Red'}])
>>> chart.add({'title': 'Violet', 
...   'xlink': 'http://en.wikipedia.org/wiki/Violet_(color)'}, 3)
>>> chart.render()


as is this PR has problems with the computation of the legend width, I could not figure out how this is computed. Also I need some guidance on how to run the tests.