BUG: Area plot legend has incorrect color

Derived from #7636. Area plot sets incorrect ``alpha`` values to legend when ``stacked=True``.

#### NG: Each areas are drawn with ``alpha=1.0``, but legend has ``alpha=0.5``
![figure_ng](https://cloud.githubusercontent.com/assets/1696302/3919509/643924e0-23a9-11e4-983d-23a16e9c01bc.png)

#### After fix
```
df = pd.DataFrame(np.random.rand(20, 5), columns=['A', 'B', 'C', 'D', 'E'])
df.plot(kind='area')
```
![figure_1](https://cloud.githubusercontent.com/assets/1696302/3919515/8b4ecf4e-23a9-11e4-926b-85101760882a.png)
```
# When alpha is specified
df.plot(kind='area', alpha=0.2)
```
![figure_2](https://cloud.githubusercontent.com/assets/1696302/3919516/8fc2c166-23a9-11e4-8ea6-b14479d6a72b.png)