Typos and such

Hi Cam,

Here are a bunch of typos I noticed while reading your book.  I separated most of the changes out into separate commits with the most controversial later so that you could easily take the first few or cherry-pick as you like.

Thanks for making this book available.  It's just what I was looking for.  I really appreciated the IPython notebook format.  I also learned as much about how to make nice looking figures in matplotlib as I did about MCMC.

-Jim