Added __getstate__ attr to cartopy.io.img_nest.Img class

This adds a `__getstate__` attribute to `cartopy.io.img_nest.Img` to ensure pickling of such objects functions correctly. By default pickle does not pick up non-default attributes that are added to an object, such as `_bbox` that is added to `Img` by its `__init__`.

This can be demonstrated with the following example:

```python
class NT(namedtuple('NT', 'foo')):
    def __init__(self, *args):
        self._bar = None


class NT2(namedtuple('NT2', 'foo')):
    def __init__(self, *args):
        self._bar = None
    def __getstate__(self):
        return self.__dict__


>>> t = NT(5)
>>> print t._bar
None
>>> u = pickle.loads(pickle.dumps(t))
>>> print u._bar
AttributeError: 'NT' object has no attribute '_bar'

>>> t2 = NT2(5)
>>> print t2._bar
None
>>> u2 = pickle.loads(pickle.dumps(t2))
print u2._bar
None
```

See http://docs.python.org/2/library/pickle.html?highlight=pickle#object.__getstate__.