Improve intro numpy

Here is the work I have been doing in preparation for the Numpy tutorial at scipy2013 on Monday 2013-06-24. It is mainly just polishing and a few features.

Feature wise, I have added short, interleaved exercises. My experience with teaching tutorials over the last few years has been, that people generally want to get their hands on the keyboard ASAP. Having many small, almost trivial exercises seems to be better than big monolithic ones at the end. Hence the longer exercises have been moved to their own section and are backup in case I finish early and for people who are getting ahead of the rest of the group. 

Also, there are three small sections that I have commented out, but not removed. Still a bit undecided what to do with them. Doctests either pass or are skipped. In addition, I have made some minor syntactic changes to allow for better conversion to IPython notebook.

Anyway, if you have any spare time, feel free to review. If I get some :thumbsup: by Sunday morning CET we might be able to squeeze a release in. Otherwise I'll direct people to my own deployment on http://scipy-lecture-notes.zetatech.org/. Incidentally, this is also where you can view the generated HTML for this branch, w/o having to checkout and build:

http://scipy-lecture-notes.zetatech.org/improve_intro_numpy/intro/numpy/operations.html

http://scipy-lecture-notes.zetatech.org/improve_intro_numpy/intro/numpy/array_object.html