`@on_trait_change` decorator is not lazy (and a special case of the same issue)

The `@on_trait_change` decorator is not lazy, which means that even if we provide a default value for a trait, the `_traitname_default` method is called on intialization.

I have two examples:

1) Consider the class

```
class B(HasTraits):
a = Any
def _a_default(self):
print 'INIT a'
return 42

>>> y=B(a=1)
```

This, as I expected, doesn't output anything.

Now if we add a `on_trait_change` decorator:

```
class A(HasTraits):
a = Any
def _a_default(self):
print 'INIT a'
return 42

@on_trait_change('a')
def _do_something(self):
print 'a CHANGED'

>>> x = A(a=1)
INIT a
a CHANGED
```

Here the 'a' default initializer is called.

2) A variant of the above behavior:

```
class X(HasTraits):
x = Float(12)

class A(HasTraits):
a = Instance(X)

def _a_default(self):
print 'INIT a'
return X()

@on_trait_change('a:x')
def _do_something(self):
print 'a:x CHANGED'

y = X(x=1)
b = A(a=y)
INIT a
```

The `_a_default` method is still called, even though the listener itself is not called.