Fix broadcast date

The text in the relevant element could be e.g. "Yesterday" rather
than a full date which isn't that useful when taken out of context.

Extract the date from an attribute that contains it instead.
Change base_URL to base_url
Handle tracks with multiple artists

The artist XPath failed when there were multiple artists.

This was a good example: http://www.bbc.co.uk/programmes/b04kzqlf

Has tracks with two and three artists. Artists are separated by commas,
except for the last one which has an ampersand preceding it.
Python 3 compatibility (hopefully!)

The only change 2to3 pointed out was this use of unicode literal.
Update docstrings to be more accurate