Speed up max_len_string_array
Before:

``` python
In [5]: x = np.array(['abcd', 'abcde', 'abcdef', 'abcdefg'] * int(1e7), object)

In [2]: f = pd.lib.max_len_string_array

In [6]: %timeit f(x)
1 loops, best of 3: 501 ms per loop
```

After:

``` python
In [1]: x = np.array(['abcd', 'abcde', 'abcdef', 'abcdefg'] * int(1e7), object)

In [2]: f = pd.lib.max_len_string_array

In [3]: %timeit f(x)
10 loops, best of 3: 68.4 ms per loop
```
