Typo fixes and proof-read of 'invocation'.

I ran all the chapters through hunspell, which found a bunch of typos and spelling mistakes, which I have corrected.  I also proof-read `invocation.html`.

Some of the fixes I've done here may be contentious.  For example, 'center' vs 'centre' - I wasn't sure if we are using en_GB or en_US.  More contentious may be 'lower case' vs 'lowercase'.  In any case, I have put each group of changes in its own commit, so you can revert any particular commit without affecting the others.

I also changed the comments to be [PEP8 compliant](http://www.python.org/dev/peps/pep-0008/#comments) - we should probably be teaching the kids to write well-styled code :)