PERF: directly astype with numpy if series is already nansafe
from [SO](http://stackoverflow.com/questions/26744370/most-efficient-way-to-convert-pandas-series-of-integers-to-strings)

so the null check is pretty cheap. if no nulls, then can just bypass nansafe an use the underlying numpy routine. should be a nice speedup.

``
In [13]: arr = np.random.randint(1,10,size=1000000)

In [14]: s = Series(arr)

In [15]: s.notnull().all()
Out[15]: True

In [16]: %timeit s.notnull().all()
1000 loops, best of 3: 1.35 ms per loop

In [17]: %timeit s.astype(str)  
1 loops, best of 3: 2.52 s per loop

In [18]: %timeit s.values.astype(str)
10 loops, best of 3: 37.7 ms per loop

```
```
