[#153] add epsilon number to avoid different result at different scale

[ptran1203]

I check your code and see that you + 1 for intersection width and height. I think you want to avoid zero division error, but it leads to a significant shift if I measure the results at different scales (image resolution).

I think we should replace 1 with a very small number such as 1e-6

https://github.com/Cartucho/mAP/blob/master/main.py#L557..L562

See Issue #153