Create an nbgitpuller "generate link" function in Python

I think another common way folks might want to be able to generate a link is via **Python** instead of via a UI (for example, if they want to generate a bunch of links programmatically, or if they are more comfortable calling a library than a UI). What do folks think about adding a `generate_link` function that takes arguments that match the fields in `nbgitpuller.link` and generates the proper link?