Feature/local asset

@plexus, that PR allows to embed locally the asset in the presentation directory. It's useful when you just copy the directory (use cases: USB key then laptop of someone else without slippery gem, rawgithub -> it's then possible to run directly the presentation)

In the two last commits, I've also fixed some doc/style issues. Code style being more a convention than an absolute truth, I would like to know if you're ok with those. If not, tell me the ones I need to revert to old version.