Updated version of exercise 2a

Note that we just updated the text for exercise 2a with a new version. Simen had noticed some unclarities that we've tried to correct, mainly in question 2b. Note also that the bulk of the work load for this problem set is associated with question 2b, as solving the overall problem here (reading in a text to populate the feature vectors) requires solving several other sub-problems (text normalization, filtering stop-words, mapping string to numerical identifiers, etc). 

(A heads-up to those who that had already started on the exercise; note that we've renamed the file with the word list to 'words.txt', and that we've singled out the function `get-feature-vector' to a separate problem, 2c).