a few misc changes

changed input back to raw_input in pluginbase. forgot to do that when I
went back from python 3

Added unicode literals to the useful module. I can't remember why.

made some changes to the pyexec plugin to try and get it working again.
I don't think I was successful.
deleted docs, added TODO section about the rewrite

I wanted to remove the docs because they were becoming more and more
out of date. Even though they still had some relevant information, I
think the code and the docstrings speak for themselves. Besides, this
isn't some project with a huge userbase, if someone is confused they can
just ask me.

I added a section to the TODO document about my rewrite ideas, and the
goals for the rewrite. I plan to add more details to that section soon.