PERF: use unique and isnull in nunique instead of value_counts.

closes #9129
 
Currently, `Series.nunique` [(source)](https://github.com/pydata/pandas/blob/master/pandas/core/base.py#L429) calls `Series.value_counts` [(source)](https://github.com/pydata/pandas/blob/master/pandas/core/base.py#L371), which by default, sorts the values. 

Counting unique values certainly doesn't require sorting, so we could fix this
by passing `sort=False` to `value_counts`.

But `nunique` can also be calculated by calling `Series.unique` instead of
`value_counts`, and using `com.isnull` to handle the `dropna` parameter.

This PR attempts to implement this. Here is a vbench perf test which seems to show an improvement for tests using `nunique`.

```
/usr/bin/time -v ./test_perf.sh -b master -t nunique-unique -r groupby
```

Here are the best and worst ratios:

```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
groupby_ngroups_10000_nunique                | 900.9844 | 3768.5087 |   0.2391 |
groupby_ngroups_100_nunique                  |   9.6850 |  38.9043 |   0.2489 |
groupby_ngroups_100_sem                      |   0.7834 |   1.4904 |   0.5256 |
groupby_ngroups_100_size                     |   0.4920 |   0.8560 |   0.5748 |
groupby_ngroups_10000_max                    |   2.5337 |   4.1683 |   0.6078 |
groupby_frame_nth_none                       |   2.3570 |   3.2880 |   0.7168 |
groupby_ngroups_10000_var                    |   2.4277 |   3.3390 |   0.7271 |
groupby_ngroups_10000_sem                    |   3.5107 |   4.6523 |   0.7546 |
...
groupby_transform_multi_key3                 | 630.2720 | 599.7047 |   1.0510 |
groupby_nth_datetimes_none                   | 424.5253 | 403.6326 |   1.0518 |
groupby_transform_series2                    | 109.7130 | 104.1580 |   1.0533 |
groupby_transform_multi_key1                 |  58.9686 |  55.7120 |   1.0585 |
groupby_nth_datetimes_any                    | 1206.7020 | 1132.4236 |   1.0656 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
```

While working on this PR I ran into issue GH 9129.
This PR includes Jeff's suggested fix.