date_range performance
Hello - can anything be done about the performance of date_range?  I've noticed a big degradation in performance from 0.11.0 (I know it's old) to 0.15.2.

_0.11.0:_

```
%timeit pandas.date_range(start='1/1/1995', end='2/3/2015', freq='B')
1000 loops, best of 3: 412 us per loop
```

_0.15.2:_

```
%timeit pandas.date_range(start='1/1/1995', end='2/3/2015', freq='B')
10 loops, best of 3: 134 ms per loop
```

Thanks
