Add unicode support to IPFS plugin

[ncryptid]

I was having an issue with the IPFS plugin not being set to encode to UTF-8 (using the default ASCII encoding in Python) and totally failing to publish if I had any albums in my library that used Unicode characters (e.g. Japanese characters). I added a small workaround to set the default encoding for this plugin to UTF-8.

This is a hacky fix, but I don't think it should really make a difference for a plugin this small. But this is my first pull request ever and I'm a total programming noob so I may have overlooked something.

[sampsyo]

Hi! Sorry, but globally changing the Python interpreter’s encoding will have lots of unintended side-effects for the rest of beets.

What’s the error you’re seeing? We have standard ways to deal with Unicode filenames (c.f. some notes on the Hacking page of the wiki), and seeing a traceback may help us give you suggestions about how to apply them.