#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)



When one wants to plot multiple histograms, I can think of two cases: 1) use different options for different histograms 2) use the same option for all histograms. If this makes sense, shall we just check if `len(style)==len(data_list)` and otherwise throw exception to cover 1)? For 2), line 996-997 will cover this. The same is applied to other options.