Fix indexing of aggregations for which axis != 0.

Aggregation.__getitem__ was behaving always as if self._axis == 0 (a previous restriction).
Problem case example: 
```
>>> data = np.zeros((5,4,3))
>>> data = biggus.NumpyArrayAdapter(data)
>>> mean = biggus.mean(data, axis=1)
>>> print 'Mean:', mean
Mean: <_Aggregation shape=(5, 3) dtype=dtype('float64')>
>>> print 'Mean[0]:', mean[0]
Mean[0]: <_Aggregation shape=(5,) dtype=dtype('float64')>
```
 -- **shape should be (3,) instead of (5,)**