Add way to use user-defined word (token) scoring function instead of the pre-baked ones.

[andrewrosss]

Right now one has to choose between one of:

• word degree
• word frequency
• word degree to frequency ratio
  it would be nice to be able to override that functionality with a custom function/method.

EDIT: One possible option here is to implement nothing, and just document the fact that overriding word_score is an option?

[andrewrosss]

Closed by #8. This can be achieved by providing a callable as the word_scorer_class argument of Rake.__init__()