Change behavior of ipython notebook --port=...

[bfroehle]

Previously ipython notebook took the port parameter as a suggested place to
start looking for open ports. With this commit:

• ipython notebook : start at port 8888 and try random ports until success
• ipython notebook --port=1234 : use port 1234, failing if it is not available

Closes gh-1750.

[bfroehle]

I first considered implementing this using another Trait:

try_other_ports = Bool(True)
def _port_changed(self, name, old, new):
    self.try_other_ports = False

But this lead to inconsistent behavior as ipython noteboook --port=8888 doesn't trigger the _port_changed function.

[ellisonbg]

Please see my recent comment on #1750. I want to maintain the ability to set the starting point of random port ranges, which my proposal would do.

[bfroehle]

I'll close this for now, to keep the discussion in #1750.