PERF: improves performance in groupby.size
on master:

``` python
In [1]: n = 100000

In [2]: np.random.seed(2718281)

In [3]: dr = date_range('2015-09-19', periods=n, freq='T')

In [4]: ts = Series(np.random.choice(n, n), index=np.random.choice(dr, n))

In [5]: %timeit ts.resample('5T', how='size')
1 loops, best of 3: 538 ms per loop
```

on branch:

``` python
In [5]: %timeit ts.resample('5T', how='size')
10 loops, best of 3: 27.6 ms per loop
```

thanks!
