Retry feature on timeout

Hi, this is not strictly a pull request :) , I'm want to learn, so if you find my proposal unfeasible, no problem!.

I was downloaded several courses through coursera-dl, however in several cases I had to be aware of the downloads because my internet is not very reliable and from time to time some downloads were cut off. So instead of writing a wish, I tried to make a patch.

That code just try few times before skip the download. Obviusly there are many other details such as I assume any exception like a timeout error.

Thanks for review it.