ENH add bins argument to value_counts
fixes #3945

Also, adds arguments present in top-level function to the Series
method (sort and ascending).

Note: this only works with numeric data (it raises a TypeError if there was a TypeError problem in cut, however perhaps cut should raise a more descirptive error...)

```
In [1]: s = pd.Series(np.random.randn(100))

In [2]: s.value_counts(bins=4)
Out[2]:
 0.017354    44
-1.154431    35
 1.189140    14
-2.330904     7
dtype: int64

In [3]: s.value_counts(bins=4, normalize=True)  # also works with sort and ascending arguments.
Out[3]:
 0.017354    0.44
-1.154431    0.35
 1.189140    0.14
-2.330904    0.07
dtype: float64

In [11]: s = pd.Series(list('abc'))

In [12]: s.value_counts(bins=1)
TypeError: bins argument only works with numeric data.
```

_Note: Have a test failing on python3 (topically) a sorting one_.
