Added support for the nifty ptpython shell.

[pvaret]

This patch adds support for the awesome ptpython shell.

[asmeurer]

That is a neat shell. I'll give it a try for a while.

Worth noting that this suffers from the same issue as ipython (#103). If you want to try to fix it, let me know. Otherwise, it looks like this works and it can be merged.

[asmeurer]

There's also ptipython (I guess the main advantage is that you get %magics).

[inducer]

Thanks for your contribution!

[inducer]

Btw, just released this as part of 2015.2.

[pvaret]

Re. ptipython: that's a GOOD point. My feeling was that the shell configuration interface is already starting to look a little crowded, and I was not sure the added value of that last option (IPython already being pretty fine on its own) was worth the clutter. It would probably not be too hard to add it as well, however.

As to the variable scoping issue, I'll look into it as time allows. Thanks for pointing it out!