added yes/no words. Modified n2w2 to use words.

In acec64fe4677305566cd2d0fa780056b07f7e6e6 there is a yes/no question
introduced in tile n2w2. This is the first time yes/no answers appear in the
game. So I added 'yes' and 'no' as words in weatbag/words.py and modified
weatbag/tiles/n2w2.py to use them, making the code a bit cleaner.
Made all yes/no words lowercase. Upper not needed.

Apparently, player input is converted to lowercase in
weatbag/action.py, so we only need lowercase words in
weatbag/words.py.
Merge pull request #23 from lathan/yesno

added yes/no words. Modified n2w2 to use words.