Fix traitlet _notify_trait by-ref issue

[jdfreder]

This fixes a problem with the existing traitlet machinery, where anytrait values get added to the notifiers list every time the _notify_trait function is called.

The bug is that each time the traitlet changes, the anytrait callbacks will each be called N times, where N is the number of times the traitlet has been changed (in the current application instance).

[Carreau]

Tests ?

For whatever reason, Travis have decided not to test this PR ...

(Yes, I know I'm the first not to write test)

[jdfreder]

Thanks for the reminder, I was still trying to isolate an example of the problem. This notebook code shows the issue:

IN

from IPython.config import LoggingConfigurable
from IPython.utils.traitlets import Int

class A(LoggingConfigurable):

    listen_to = ['a']

    a = Int(0)
    b = 0

    def __init__(self, **kwargs):
        super(A, self).__init__(**kwargs)
        self.on_trait_change(self.listener1, self.listen_to)

    def listener1(self, name, old, new):
        self.b += 1

class B(A):

    c = 0
    d = 0

    def __init__(self, **kwargs):
        super(B, self).__init__(**kwargs)
        self.on_trait_change(self.listener2)

    def listener2(self, name, old, new):
        self.c += 1

    def _a_changed(self, name, old, new):
        self.d += 1

IN

instance = B()

for i in range(3):
    instance.a += 1
    print "%d should equal %d and %d" % (instance.b, instance.c, instance.d)

OUT

1 should equal 1 and 1
2 should equal 3 and 3
3 should equal 6 and 6

[jdfreder]

Test added