ZIP support and get_series network optimisation

I haven't tested this widely, but I made these changes for a personal project so I thought I'd share them to this branch if you were interested.

Firstly, and item from the TODO, I've added zip support for pulling the .zip data. I currently don't unzip and buffer the extra information in the zip, so this could be done for further performance.

I've also noted that get_series already gets all the season and episode data in its request. This information is now stored in `full_data` and used by `_populate_data` rather than making another request when available.