Introduce a variable for the path to the default font "league_gothic"

Hi there,

I would like to integrate the included `reveal.js`-themes better with [`middleman-presentation`](https://github.com/maxmeyer/middleman-presentation):

The path to the font in the default theme is hardcoded. I'm using `middleman` + `sprockets` + `bower` to serve the presentation and other assets. `reveal.js` is installed via `bower` as well. All assets in there go through `sprockets` and `bower`

With the path to the `reveal.js` default font (`league_gothic`) hardcoded I see no way to serve it via middleman. Therefor I introduced a scss variable (`$rv-font-path`) to make the path to the font configurable. With this in place, it is possible to serve the default font via `middleman`.

To make the use of fonts easier and to reduce redundancy I also introduced a small mixin: `rv-font-face`. Addititionally I moved the font files into a subfolder `league_gothic` to separate them from other font files which might be included by a user in their presentation.

BTW: I compiled the `scss`-files and they looked very similar to the existing `css`-files.  These compiled `css`-files are *not* part of the pull request. If you like, I can include them as well.

Would you mind to take a look into this ? Thank you very much in advance.

Cheers,
maxmeyer