snooping on descriptors

Currently it fails with an error:

```python
import snoop

class Test:
# TypeError: compile() arg 1 must be a string, bytes or AST object
@snoop
@classmethod
def class_test(cls):
pass
```

Workaround:
```python
import snoop

class Test:
@classmethod
@snoop
def class_test(cls):
pass
```

Injecting snoop to a classmethod at runtime:
```python
Test.class_test = classmethod(snoop(Test.class_test.__func__))
```