CH1: misc fixes

Hi,

I've read throough CH1 and here are some suggested fixed.
You're welcome to take or drop whatever you like, ofcourse.
The inlined plots for the posteriors don't seem to match the code
anymore, rerunning the code produced poor results.
I made some tweaks, and I hope you'll merge at least that in:


![am](https://f.cloud.github.com/assets/1820866/268164/6f8650a0-8f0f-11e2-90c2-18c01fbdcbab.png)


I omitted the image data, so changes won't show up until the cells are rerun.


Thank you so much for making this available,  really looking forward to CH2.