More descriptive ylabel for Series.hist()

```
s = pd.Series(np.random.randn(100))
s.plot(kind='hist')
s.hist()
```

Running `s.plot(kind='hist')` labels the y axis with the word 'Degree'. A more informative label might be 'count', 'frequency', or 'density'.

Also, it seems strange that `s.plot(kind='hist')` and `s.hist()` produce plots with different labels / gridlines.

Looks like this came from PR #7809. cc @sinhrks, @jreback
