Added comma condition to PunktWordTokeniser

This addition to the word tokeniser splits words that are separated by commas but not numbers. For example the sentence:
    'I have 13,000,000 bottles,in,my,bag'
tokenises as:
    ['I', 'have', '13,000,000', 'bottles', ',', 'in', ',', 'my', ',', 'bag']
rather than the awkward:
    ['I', 'have', '13,000,000', 'bottles,in,my,bag']

This is useful as this occurs quite a lot in blogs and tweets and has caused errors in my processing chain when we had some 1600 words joined in this way - leading to a massive word being created...