Added test for legends on scatter plots

I am encountering a bug very similar to #86 and #48 when using a scatter plot, a legend, and tooltips.  The legend box is shown but the markers for the different data series are not shown.  I can recreate it by making the changes that @gabraganca made to the [tooltip example](http://mpld3.github.io/examples/scatter_tooltip.html#scatter-tooltip).  
![image](https://cloud.githubusercontent.com/assets/727375/4921006/0090c0e8-6507-11e4-970b-1baee08ffbc4.png)

It also occurs if I modify `test_plots/test_legend.py` to use scatter plots, even without tooltips.
This pull request creates `test_plots/test_legend_scatter.py` which creates similar plots using `plt.scatter`.
![image](https://cloud.githubusercontent.com/assets/727375/4921412/477708f2-650a-11e4-8227-dd7202b2945f.png)