#4

Now HistogramSysTest.HistoPlot accepts either a numpy ndarray or a list of ndarray as input, and will overplot the histograms together is multiple datasets are provided.  

Weighted-histogram is also allowed.

![gaussian_test_1](https://cloud.githubusercontent.com/assets/7539807/4351306/ea0b5368-4208-11e4-9af4-c86d1b1ad67f.png)
![gaussian_test_2](https://cloud.githubusercontent.com/assets/7539807/4351307/ea172620-4208-11e4-90ae-9084e2f6a9f9.png)
![gaussian_test_3](https://cloud.githubusercontent.com/assets/7539807/4351308/ea190bc0-4208-11e4-9638-2d62a90a996e.png)



Since you do this same bit of code many times, it might make sense to write it as a separate helper function. (Then it's easier to fix if we decide to change something later.)