Indexing pd.Series much slower than indexing dict
I noticed that indexing via dict is ~100 times faster than through `Series` in 0.14.1. Is this known (to be expected)? 

```
mydict = {'a': 3, 'b': 4}
myser = pd.Series(mydict)
%timeit mydict['a']
%timeit myser['a']
10000000 loops, best of 3: 39.4 ns per loop
100000 loops, best of 3: 4.9 µs per loop
```
