ENH/VIS: Area plot is now supported by kind='area'. 

Area plot is added to plotting method. The AreaPlot class is created as a subclass of LinePlot, thus it works also in time series. 

By default, area plot is being stacked. When area plot is not stacked (`stacked=False`), alpha value is set to 0.5 to show overlapped area if not configured specifically. As a side benefit, line plot also can be stacked by specifying `stacked=True` (disabled by default). Different from stacked bar plot, I don't know good visualization for positive/negative mixed data. Thus, input must be all positive or all negative when `stacked=True`. I'll try to implement it if there is a good way. Also, area plot doesn't support logy and loglog plot because filling area starts from 0.

Note: Area plot's legend is implemented based on the answer described in:
http://stackoverflow.com/questions/14534130/legend-not-showing-up-in-matplotlib-stacked-area-plot

*Example:*
![figure_1](https://f.cloud.github.com/assets/1696302/2436288/dee8f4e6-add8-11e3-9297-a7403935844d.png)