Feature: configurable position & font size

Implemented suggestion from https://github.com/josevalim/rails-footnotes/issues/105

It should be pretty self explanatory based on what I've added to the readme:

You can also lock the footnotes to the top of the window, hidden by default, and accessible
via a small button fixed to the top-right of your browser:

    f.lock_top_right = true

To set the font-size for the footnotes:

    f.font_size = '13px'

--

Also, I've changed the font to a monospace font, which I think is better for readability (rather than inheriting the default body font). I was going to make font_family a configurable option but decided against it - let me know if you'd like me to add that to this PR.

Lastly, I haven't added any tests... I noticed other simple features like `@@no_style` aren't tested. If you want tests, please give some indication of what you think I should test for - I'm newish to ruby / rails.