#362 Sample operation initial work

Initial version of Sample() operation implemented and tested for numpy and pandas.
Sample() should randomly select some items from a given datasource and return them.
For numpy and pandas, the returned data are new arrays or dataframes or series.

@mrocklin - Do I understand correctly that the input data would then have to be converted to/from np.ndarray upon entry and exit?
Something like the following?
```python
f = compute_one.dispatch(Sample, np.ndarray)
return into(originating_type, f(expr, into(np.ndarray, data), **kwargs)), **kwargs)
```