Added putioDownloader

I have added one file couchpotato/core/downloaders

This file is a downloader for put.io.  I have tested and it does work, though it could use some work.  I read the docs on how to add stuff to couchpotato, but it seems to be related to people who want to fix bugs, in this case I'm not fixing a bug I'm adding a new feature...

Let me know if you need more information.

It does seem that this is a feature requested in #3079
I've fixed that issue, and committed it, do I need to create a new pull request?