Fix an observe-decorated _name_changed method

[mdickinson]

This PR changes the behaviour of a method that's decorated with observe but also has the magic _somename_changed naming.

Prior to this PR, such a method would be registered twice by the Traits machinery, and called once with a TraitChangeEvent and once with the new value of the trait:

>>> from traits.api import *
>>> class A(HasTraits):
...     foo = Int()
...     @observe("foo")
...     def _foo_changed(self, arg):
...         print(arg)
... 
>>> a = A()
>>> a.foo = 23  # Expect one call to `print`; get two calls.
23
TraitChangeEvent(object=<__main__.A object at 0x10ad14450>, name='foo', old=0, new=23)

With this PR, it will only be called once, with the TraitChangeEvent: the observe decorator supersedes and nullifies the magic naming. This is the same behaviour that's already present for on_trait_change:

>>> from traits.api import *
>>> class A(HasTraits):
...     foo = Int()
...     @observe("foo")
...     def _foo_changed(self, arg):
...         print(arg)
... 
>>> a = A()
>>> a.foo = 23  # Get one call to `print`, as expected.
TraitChangeEvent(object=<__main__.A object at 0x1056994a0>, name='foo', old=0, new=23)

I don't think we need a deprecation period for the change in behaviour: I doubt that there's working code that relies on the method being called twice with different arguments for each call, so I think this qualifies as a bugfix.

The PR also adds tests that exercise the corresponding code for on_trait_change.

Closes #527

[jwiggins]

LGTM