Finish up new numpy section

[GaelVaroquaux]

• In operations.rst, around line 209: the "data statistics" example
  can probably be completely removed in my eyes (it is in the
  exercises.rst file).
• I'd really like the random walk example back in. I use it to teach
  how a problem can be vectorized, rather than relying on for loops.
• The transpose remark should be put back in. It is a very important
  remark, in my experience.
• array_object.rst, line 767: I would keep the simple 'arange(10)'
  example, just for the sake of simplicity, unless I am missing the point.

[GaelVaroquaux]

Make the transpose issue more explicit. I would explicitly add: don't do: "a += a.T".

[pdebuyl]

1. PR coming
2. You (@GaelVaroquaux ) already put the example back in :-)
3. The transpose example is correct since several versions of NumPy, see 'in-place operation' code example obsolete? #278
4. I have no idea where to look at in the current source file.

[pdebuyl]

To close up, I think we should merge the PR and remove the transpose warning.