Introduce a variable for the path to the default font "league_gothic" based on #907

Hello @hakimel,

I fixed the "branch" problem and created a new PR instead of #907. I will close #907.

Original message:
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Hi there,

I would like to integrate the included reveal.js-themes better with 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