ENH add sample #2419
fixes #2419

Hmmm `np.random.choice` not available on numpy < 1.7.

closing for now. @hayd if you want to update at some point, ok,.
 Just wrote a little utility function to do this, so I may pick this up.

``` python
def sample_n(df, size, replace=False, weight=None, seed=None):
    rs = np.random.RandomState(seed)
    locs = rs.choice(df.shape[0], size=size, replace=replace, p=weight)
    return df.take(locs, axis=0)
```
