Add WordsTable class
Add Generator#increment and #include?
Move Generator#heard_words to WordsTable#words

* Add private WordsTable#add_row
Add WordsTable#words_after
Make Generator use WordsTable
Clean up WordsTable#words_after
Allow WordsTable#increment with no args
Merge pull request #6 from jakeboxer/words-table

Extract words table stuff from Generator into its own class