SparseSeries.__add__ perf bad with large # of blocks
``` python
In [44]: foo = Series(np.random.randn(100000))

In [45]: bar = Series(np.random.randn(100000))

In [46]: foo[::3] = np.nan

In [47]: bar[::4] = np.nan

In [48]: sfoo = foo.to_sparse(kind='integer')

In [49]: sbar = bar.to_sparse(kind='integer')

In [52]: timeit sfoo + sbar
100 loops, best of 3: 15.8 ms per loop

# implicitly kind='block
In [53]: sbar = bar.to_sparse()

In [54]: sfoo = foo.to_sparse()

In [55]: timeit sfoo + sbar
1 loops, best of 3: 217 ms per loop
```
