Add command extraction for reply texts.

This works, but I don't know how robust it is. So it looks like there
are two ways that you can communicate to the shubot email account from
my phone. I can 1) email a picture message (which is annoying and extra
work than I usually do) or 2) reply to a message that shubot has sent
me, which requires shubot to have sent me an email. I'm leaning towards
2) because it's most fluid if shubot has sent me an email recently. It
could work as a sort of authentication/token type of thing too. I tried
replying to an old message on my phone sent from shubot and it failed to
send. I assume these messages are sent to t-mobile who receives them and
then sends them off to me, but the number it recieves them on seems to
rotate. Not sure when a rotation occurs, which will affect the annoyance
factor of 2).

But for now, I can reply and check_mail.py can get out the text that has
been sent. This may be enough if only I end up using shubot. I imagine
with other phone companies, it'll be a whole different ballgame.