Avoid stemming short words

Lots of 2-letter abbreviations are used on twitter. Many end on 's',
which the stemmer removes, making it impossible to distinguish
different abbreviations.
Add support for more emoticons
Add support for better apostrophe-d abbrevations such as don'ts and gov'ts
Add support for fractions (3/4s) and abbreviations with slashes ("financial f/cs")
Compile bug in TwitterStats class.