_repr_html_ returns None 
In previous versions, the Spyder IDE displayed HTML for DataFrames in its iPython console. In 0.15.2 they are no longer displayed.

I think this may be to do with `DataFrame._repr_html_()`

If I run the following on 0.15.2:

```
import pandas as pd
import numpy as np

print('pandas version ', pd.__version__)
rng = pd.date_range(start='2014-01-01', periods=5, freq='D')
df = pd.DataFrame({'y':np.random.normal(size=len(rng))}, index=rng)
ret = df._repr_html_()
print(None == ret)
```

it prints `0.15.2, True`

and viewing the `DataFrame` with `df` results in the text rendering:
![image](https://cloud.githubusercontent.com/assets/10627907/5833104/1239147c-a147-11e4-8991-4f363490ec35.png)

On 0.14.1, running the above I get `0.14.1, False`
and  `df` results in this:
![image](https://cloud.githubusercontent.com/assets/10627907/5833089/d315dbfe-a146-11e4-8197-95b37af9add9.png)
