add in-memory processing of images from url

[stephenhu]

thanks for creating this library, i realize the library's detection of nudity is not high after running through some samples and reading the issues, but fwiw i had modified the code already before looking at the results/comments so i'm providing this code for you. basically this allows you to take images from urls, save to memory and detect nudity without storing to disk.

i plan on leveraging go-cv to create a nudity detection algorithm, will let you know when i've completed if interested.