Deprecate BeautifulSoup 3

Deprecate Beautiful Soup 3 in favour of the Python3-compatible Beautiful Soup 4.

The XML parsing capabilities of BS3 will be replaced with the [The ElementTree XML API](http://docs.python.org/2/library/xml.etree.elementtree.html)

Tasks:
- [x] `utils/imdb.py`
- [x] `plugins/api_tvdb.py`
- [x] `plugins/input/thetvdb_favorites.py`