Dynamically find port

[ijstokes]

When doing conda launch foo, start up a new app server as the default behavior for each launch. This will require dynamic port assignment. Borrow the strategy from IPython.

[asmeurer]

I already ported out the code from IPython for this in anaconda-launcher. See https://github.com/ContinuumIO/anaconda-launcher/blob/master/launcher/main.py#L331 (it actually comes from tornado, not IPython directly).

[ijstokes]

Awesome, thanks. I'll use that.