PERF: nunique perf improved by using len(unique) rather than value_counts
Before:

```
In [1]: s=pandas.Series(range(100)*1000)
In [2]: %timeit s.nunique()
1000 loops, best of 3: 1.43 ms per loop
```

After:

```
In [1]: s=pandas.Series(range(100)*1000)
In [4]: %timeit s.nunique()
1000 loops, best of 3: 440 µs per loop
```
