NaN equality issue in Series
Hi Wes,

Just a quick question on NaN equality, I haven't specifically looked it up, but upgrading to 0.7.2 broke my code and it seems it's down to how testing for equality with NaN behaves:

```
a = pandas.Series(['abc', nan])
```

a == 'cba'    outputs:

```
0    False
1      NaN
```

rather than all False. This seems to most definitely be a bug. Especially as 

pandas.DataFrame(['abc', nan]) == 'cba'    outputs the correct version:

```
       0
0  False
1  False
```

Many thanks,
Marius
