Cleaning the repo

Well, I couldn't find how to reopen a pull request or something like that, so I'm opening another one.

The idea is basically the same: removed `test/` and rewrote parts of the README. I also changed the LOAD_PATH in bin/sciruby-plotter, so it'll require the "local" sciruby script, not the installed gem. And apparently some RSpec options were removed, so I edited .rspec as well.

Ah, I'm not sure how we should test SciRuby::Plotter, that's why I didn't include tests.

I hope it's good to merge now! :)