Issue547: fixes pysh profile and creates pysh extension

This is my proposed fix for a very old issue: https://github.com/ipython/ipython/issues/547


This gives me a sense of foreboding. What if I want to call a variable `date`, `dir` or `diff`? All of those are commands on my system.

To be honest, given how long we've gone without the pysh profile, I'm inclined to conclude that not that many of our users miss it, and it's not worth reintegrating into core IPython. Why not maintain this extension/profile separately? We'd be happy to list it in the extensions index. We're planning to split the project up into smaller pieces after 3.0 anyway, so this seems like the wrong time to be integrating things like this into the repository.