`mc` -> `pm` and a bunch of typos

Currently done with Chapters 2,3,4 Will update this PR while reading the book to the end. You can merge when I'm done.

- [x] chapter 2
- [x] chapter 3
- [x] chapter 4
- [x] chapter 5
- [x] chapter 6

fixes #144

- As pymc is now imported as pm, what about pymc.plot as mcplot? should it be pmplot now?
- Also found that the last headline in chapter 3 has no content. "Covariance matrices and eliminating parameters" Bug? or just not finished?