Editex groupings not correct

[alexanderamy]

Use of one-to-one mapping from chars to ints as implemented by letter_groups dict on lines 235-246 of editex.py does not allow for chars to be in multiple groups. For example, Z, which should be in groups 8 and 9 per page 4 of Phonetic String Matching: Lessons from Information Retrieval is only in group 9 in current implementation.