implement (slow) seed expansion for small seeds

where small is defined as < depth tokens.
implement a more general improveSeed

that drops elements until it finds something interesting to say.  If it
drops < depth, it delegates to expandSeed, which may drop all the way to
0 and select a random init.
improve the specs for improveSeed
use the text preceding the bot's name as the seed
don't send ! commands, ever
Merge pull request #12 from jayferd/feature.seed-expansion

implement (slow) seed expansion for small seeds