[WIP] More documentation minor fixes

A PR I plan to keep open while I am browsing the documentation and bumping into minor things that can be improved.
This is the most controversial change. The --pylab flag will be removed for IPython **notebook** on IPython 3.0 and discouraged for other front-ends. Not sure about bog-standard IPython to be entirely fair, but I tend to agree with the general opinion that putting so many things in the main namespace (and shadowing builtins while we are at it like sum and all) is never a great idea.

On top of that I didn't look extensively but I could not find a place in the doc that actually relied on accessing functions that  --pylab puts in the main namespace.