PERF: perf improvements in drop_duplicates for integer dtyped arrays
no need to factorize integers when dropping duplicates.

master

```
In [1]: np.random.seed(1234)

In [2]: df = DataFrame({'A' : np.random.randint(0,10000,size=1000000)})                

In [3]: %timeit df.drop_duplicates()
10 loops, best of 3: 36.9 ms per loop
```

PR

```
In [2]: df = DataFrame({'A' : np.random.randint(0,10000,size=1000000)})

In [3]: %timeit df.drop_duplicates()
10 loops, best of 3: 21.6 ms per loop
```
