Add support for bpython.

bpython is a fancy interface to the Python interpreter, with lots of features like syntax highlight (using pygments), autocomplete and pastebin support.

This is a trivial patch, we might rethink the `start_python_console` arguments in order to let the user choose which console the user wants. This code defaults to bpython if it is available, falls back to IPython and, finally, python if no fancier interpreter is around.