Change secret token

There were no sessions before, so who cares if it changed (it did). This is set
as an environment variable, which is the correct way to do this if you're going
to have a public repo.
Sentiment analysis

Check the last 10 tweets that mention caltrain and see if they were overall
positive or negative. This is a very naive approach, but I expect it will
give some interesting data points.
Show last 10 #caltrain tweets

Also, added some different messages depending on the sentiment.

Pretty happy with how quickly this all came together.